add contributing guide

CONTRIBUTING.md

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+# Contributing to Terraform
+
+**First:** if you're unsure or afraid of _anything_, just ask
+or submit the issue or pull request anyways. You won't be yelled at for
+giving your best effort. The worst that can happen is that you'll be
+politely asked to change something. We appreciate any sort of contributions,
+and don't want a wall of rules to get in the way of that.
+
+However, for those individuals who want a bit more guidance on the
+best way to contribute to the project, read on. This document will cover
+what we're looking for. By addressing all the points we're looking for,
+it raises the chances we can quickly merge or address your contributions.
+
+## Issues
+
+### Reporting an Issue
+
+* Make sure you test against the latest released version. It is possible
+  we already fixed the bug you're experiencing.
+
+* Provide your your `.tf` files, with secrets removed, so we can
+  try to reproduce it. Without this, it makes it much harder to fix
+  the issue.
+
+* If you experienced a panic, please create a [gist](https://gist.github.com)
+  of the *entire* generated crash log for us to look at. Double check
+  no sensitive items were in the log.
+
+* Respond as promptly as possible to any questions made by the Terraform
+  team to your issue. Stale issues will be closed.
+
+### Issue Lifecycle
+
+1. The issue is reported.
+
+2. The issue is verified and categorized by a Terraform collaborator.
+   Categorization is done via tags. For example, bugs are marked as "bugs".
+
+3. Unless it is critical, the issue is left for a period of time (sometimes
+   many weeks), giving outside contributors a chance to address the issue.
+
+4. The issue is addressed in a pull request or commit. The issue will be
+   referenced in the commit message so that the code that fixes it is clearly
+   linked.
+
+5. The issue is closed. Somtimes, valid issues will be closed to keep
+   the issue tracker clean. The issue is still indexed and available for
+   future viewers, or can be re-opened if necessary.
+
+## Setting up Go to work on Terraform
+
+If you have never worked with Go before, you will have to complete the
+following steps in order to be able to compile and test Terraform.
+
+1. Install Go. Make sure the Go version is at least Go 1.2. Terraform will not work with anything less than
+   Go 1.2. On a Mac, you can `brew install go` to install Go 1.2.
+
+2. Set and export the `GOPATH` environment variable and update your `PATH`.
+   For example, you can add to your `.bash_profile`.
+
+    ```
+    export GOPATH=$HOME/Documents/golang
+    export PATH=$PATH:$GOPATH/bin
+    ```
+
+3. [Follow the development guide](https://github.com/hashicorp/terraform#developing-terraform)
+
+5. Make your changes to the Terraform source, being sure to run the basic
+   tests.
+
+7. If everything works well and the tests pass, run `go fmt` on your code
+   before submitting a pull request.