Mentioned release guide.

Branching and tagging for releases will be in release process guide (does not exist yet).
Added formal variation for dedicated project developers.

This addresses the comment raised by Hugh Sorby on the original pull request.
More info on commit message structure.

Richard suggested using MSVTK, but The Ruby On Rails example included more info in an easier presentation.
Corrected literal block syntax.
Whitespace fixes
Was repetitive.
Rephrasing to clarify when/why pull/issue are optional/mandatory
Discarded separate pull/merge for test/implementation.

I really doubt that is a good and workable process. Pretty sure overheads outweigh the benefits.
Probably best if some other project tries that first, but it seems out-of-place for this stage of the libCellML project.
It is preferable to just have the reviewers check that test coverage is sufficient.
Minor: the word "process" was missing.