Docs: Simplify README (#19702)

* Simplify README

* Fix review comments

* Rename Developer guide

contribute/developer-guide.md

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+# Developer guide
+
+This guide helps you get started developing Grafana.
+
+## Dependencies
+
+Make sure you have the following dependencies installed before moving on to set up your developer environment:
+
+- [Git](https://git-scm.com/)
+- [Go](https://golang.org/dl/)
+- [Node.js (Long Term Support)](https://nodejs.org)
+- [Yarn](https://yarnpkg.com)
+
+### macOS
+
+We recommend using [Homebrew](https://brew.sh/) for installing any missing dependencies:
+
+```
+brew install git
+brew install go
+brew install node
+
+npm install -g yarn
+```
+
+## Download Grafana
+
+We recommend using Go to download the source code for the Grafana project:
+
+1. Add `export GOPATH=$HOME/go/` to the bottom of your `$HOME/.bash_profile`.
+1. Open a terminal and run `go get github.com/grafana/` in your terminal. This command downloads, and installs Grafana to your `$GOPATH`.
+1. Open `$GOPATH/src/github.com/grafana/grafana` in your favorite code editor.
+
+## Build Grafana
+
+Grafana consists of two components; the _frontend_, and the _backend_.
+
+### Frontend
+
+Before we can build the frontend assets, we need to install the dependencies:
+
+```
+yarn install --pure-lockfile
+```
+
+After the command has finished, we can start building our source code:
+
+```
+yarn start
+```
+
+Once `yarn start` has built the assets it will continue to do so whenever any of the files change. This means you don't have to manually build the assets whenever you've made a change to the code.
+
+Next, we'll build the web server that will serve the frontend assets we just built.
+
+### Backend
+
+Build and run the backend, by running `make run` in the root directory of the repository. This command will compile the Go source code, and start a web server.
+
+By default, you can access the web server at `http://localhost:3000/`.
+
+Log in using the default credentials:
+
+| username | password |
+| -------- | -------- |
+| `admin`  | `admin`  |
+
+When you log in for the first time, Grafana will ask you to change your password.
+
+## Test Grafana
+
+The test suite consists of three types of tests: _Frontend tests_, _backend tests_, and _end-to-end tests_.
+
+### Run frontend tests
+
+We use [jest](https://jestjs.io/) for our frontend tests. Run them using yarn:
+
+```
+yarn jest
+```
+
+### Run backend tests
+
+If you're developing for the backend, run the tests with the standard Go tool:
+
+```
+go test -v ./pkg/...
+```
+
+### Run end-to-end tests
+
+The end-to-end tests in Grafana uses [puppeteer](https://github.com/GoogleChrome/puppeteer) to run automated scripts in a headless Chrome browser. To run the tests:
+
+```
+yarn e2e-tests
+```
+
+By default, the end-to-end tests assumes Grafana is available on `localhost:3000`. To use a specific URL, set the `BASE_URL` environment variable:
+
+```
+BASE_URL=http://localhost:3333 yarn e2e-tests
+```
+
+To follow the tests in the browser while they're running, add the `BROWSER` and `SLOWMO` environment variables:
+
+```
+BROWSER=1 SLOWMO=1 yarn e2e-tests
+```
+
+## Configure Grafana for development
+
+The default configuration, `grafana.ini`, is located in the `conf` directory. 
+
+To override the default configuration, create a `custom.ini` file in the `conf` directory. You only need to add the options you wish to override.
+
+Enable the development mode, by adding the following line in your `custom.ini`:
+
+```
+app_mode = development
+```
+
+
+### Add data sources
+
+By now, you should be able to build and test a change you've made to the Grafana source code. In most cases, you need to add at least one data source to verify the change.
+
+To set up data sources for your development environment, go to the [devenv](devenv) directory in the Grafana repository:
+
+```
+cd devenv
+```
+
+Run the `setup.sh` script to setup a set of data sources and dashboards in your local Grafana. The script creates a set of data sources called **gdev-\<type\>**, and a set of dashboards located in a folder called **gdev dashboards**.
+
+Some of the data sources require databases to run in the background.
+
+Installing and configuring databases can be a tricky business. Grafana uses [Docker](https://docker.com) to make the task of setting up databases a little easier. Make sure you [install Docker](https://docs.docker.com/docker-for-mac/install/) before proceeding to the next step.
+
+In the root directory of your Grafana repository, run the following command:
+
+```
+make devenv sources=influxdb,loki
+```
+
+The script generates a Docker Compose file with the databases you specify as `sources`, and runs them in the background.
+
+See the repository for all the [available data sources](https://github.com/grafana/grafana/tree/master/devenv/docker/blocks). Note that some data sources have specific Docker images for macOS, e.g. `prometheus_mac`.
+
+## Build a Docker image
+
+To build a Docker image, run:
+
+```
+make build-docker-full
+```
+
+The resulting image will be tagged as grafana/grafana:dev.
+
+**Note:** If you've already set up a local development environment, and you're running a `linux/amd64` machine, you can speed up building the Docker image:
+
+1. Build the frontend: `go run build.go build-frontend`.
+1. Build the Docker image: `make build-docker-dev`.
+
+**Note:** If you are using Docker for macOS, be sure to set the memory limit to be larger than 2 GiB. Otherwise `grunt build` may fail. The memory limit settings are available under **Docker Desktop** -> **Preferences** -> **Advanced**.
+
+## Learn more
+
+- Read our [style guides](/contribute/style-guides).
+- Learn how to [Create a pull request](/contribute/pull-request.md).
+- Read [How to contribute to Grafana as a junior dev](https://medium.com/@ivanahuckova/how-to-contribute-to-grafana-as-junior-dev-c01fe3064502) by [Ivana Huckova](https://medium.com/@ivanahuckova).