Update HACKING file to refer to --enable-debug and sep build dir

Using a separate build dir helps contributors be less confused
as to what is a file that can be edited and what will be
overwritten in the next build.

--enable-debug disables minify and so on.

Add a section about making changes and contributing.

Closes #488
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>