Add build system documentation.

[bradleysepos]

We used to have documentation about the build system itself in HandBrake's doc directory (see attached). It needs to be revised and integrated into the developer documentation here.

doc.zip

[StatusCode404]

There is this documentation from a mirror of an old Handbrake from a decade ago that's the same as the one in the zip that I've used as reference for doing custom CFLAGS for default and each CONTRIB module. Amazing that it still works!!! Works 100% completely to emphasise that!
https://github.com/griff/HandBrake/blob/master/doc/BUILD-Linux

I documented how to set CFLAGS for default and select CONTRIB modules here: HandBrake/HandBrake#1072 (comment)

YES it would be great if a newer re-hashed version of that is added in the same old "doc" directory.