Rename the README and fix it up to match modern reality.

pkg/mac/README

@@ -0,0 +1,42 @@
+Building pgAdmin4.dmg on Mac OS X
+=================================
+
+Required Packages (Either build the sources or get them from macports or
+similar):
+
+1. Python installation
+  - Python 3.4+ or above from https://www.python.org/
+
+2. QT installation
+  - Qt 5 from http://www.qt.io/
+
+3. PostgreSQL installation
+  - PostgreSQL 9.5 or above from http://www.postgresql.org/
+
+Building:
+
+1. Set the PYTHON_HOME environment variable to the Python root installation
+   directory, e.g.
+
+   export PYTHON_HOME=/opt/local
+
+2. Set the QTDIR environment variable to the QT root installation directory,
+   e.g.
+
+   export QTDIR=~/Qt/5.14.2/clang_64
+
+3. Set the PGDIR environment variable to the PostgreSQL installation directory,
+   e.g.
+
+   export PGDIR=/usr/local/pgsql
+
+4. Copy framework.conf.in to framework.conf, and edit the values accordingly.
+
+5. If you want to codesign the appbundle, copy codesign.conf.in to
+   codesign.conf and set the values accordingly.
+
+6. To build, go to pgAdmin4 source root directory and execute "make appbundle".
+   This will create the python virtual environment and install all the required
+   python modules mentioned in the requirements file using pip, build the
+   runtime code and finally create the app bundle and the DMG in ./dist
+   directory.