Change the Krita Docs Contributors Readme to be a bit clearer on protocol.

This changes primarily the Contributor's Readme for the manual.

I feel like it is really important that we hash out protocol/workflow so that we have something to fall back on.

Subjects include:

• Tagging/Branch workflow
• Adding/Removing/Changing pages
• What to do with mistakes/missing pages
• Proofreading
• Vestigial Translation section.

And I adjusted some stuff in the mark-up conventions section.

Differential Revision: https://phabricator.kde.org/D13169