Include guidelines for adding diagrams/graphics source files

[sjay72]

Recommendation: Community projects should possess and maintain source files for any diagrams or images, whether they are used for internal discussions, in the upstream documentation, or as the "mock-up" for the creation of RH-style images created by CCS for use downstream.

Scenario that inspired this guideline:

• Devs created diagrams to use in the upstream documentation, but did not add source files to the repo.
• A Dev shared the .png "output" with a CCS writer for modification by a RH graphics person.
• CCS writer contributed the RH-style image ouput back to the Community (sans all indication of being touched by RH)
• Devs now want to iterate on the diagrams for functionality they're adding to their project/product. However, they are starting from scratch, as no source of the originals was ever added to the Community repo.
• Providing source from RH-created diagrams is probably not viable due to tools-compatibility reasons. And may have legal concerns around that, but I'm not sure.

[pmkovar]

Should also add that the generally preferred source format for vector graphics is SVG.