Contributing manual GPAC outputs

[DenizUgur]

Since with the addition of manualDump property to the GPAC output files we need to ensure these files (or all of them) conforms to a schema that our scripts can understand.

This schema is almost identical to the one we did for extension files.

[DenizUgur]

Also add an option to add this flag automatically (per this request). Also, warn the user since this would effectively eliminate the automation benefits (updating the output with newer version of GPAC)

[DenizUgur]

Update CONTRIBUTING.md accordingly. Effectively closes #95