Update other plain-text documentation to point to OpenPrinting.

Update other plain-text documentation to point to OpenPrinting.

CHANGES.md:
- Add release date, next release is 2.4rc1, per CMP.

CONTRIBUTING.md:
- No more Apple contribution agreement.

CREDITS.md:
- Direct corrections to Github issue tracker instead of my old EMail.

DEVELOPING.md:
- Relax C99/C++ comment prohibition.
- Update file naming standard (no more 16 character limit, but specify the
  allowed characters)
- Update examples to show the use of // comments and new copyright notice.
- Note that some markdown can be used in comments.