Rename 'HACKING' to 'CONTRIBUTING'

And remove redundant and obsolete information.

CONTRIBUTING.md

@@ -0,0 +1,40 @@
+If you want to hack on the GTK+ project, you'll need to have the development
+tools appropriate for your operating system, including:
+
+	- Python (2.7 or 3.x)
+	- Meson
+	- Ninja
+	- Gettext (19.7 or newer)
+	- a C99 compatible compiler
+
+Up-to-date instructions about developing GNOME applications and libraries
+can be found here:
+
+        https://developer.gnome.org
+
+Information about using git with GNOME can be found here:
+
+        https://wiki.gnome.org/Git
+
+In order to get Git GTK+ installed on your system, you need to have the
+required versions of all the GTK+ dependencies; typically, this means a
+recent version of GLib, Cairo, Pango, and ATK.
+
+**Note**: if you plan to push changes to back to the master repository and
+have a GNOME account, you want to use the following instead:
+
+```sh
+$ git clone ssh://<username>@git.gnome.org/git/gtk+
+```
+
+To compile the Git version of GTK+ on your system, you will need to
+configure your build using Meson:
+
+```sh
+$ meson _builddir .
+```
+
+For information about submitting patches and pushing changes to Git, see the
+`README.md` and `README.commits` files. In particular, don't, under any
+circumstances, push anything to Git before reading and understanding
+`README.commmits`.