Document contribution guidelines aren't consistent

[robertwray]

There are two places within the repository where documentation contribution guidelines are covered:

• Contributing.md in the root of the repository
• Contributing-documentation.markdown in docs/documentation

The content in .md is the more comprehensive of the two, with the .markdown file containing an instruction to add a "targets and tested against version X" top-hat to documentation pages and nothing more. The dissonance between these two documents (as it's not even a case that one is a subset/superset of the other) should be resolved.

[robertwray]

Proposals/Options:

1. Add the "version top-hat" note to contributing.md and then replace the content of Contributing-documentation.markdown with that of contributing.md
2. Add the "version top-hat" note to contributing.md and then replace the content of Contributing-documentation.markdown with a reference & link to contributing.md

My preference is for option 2 as this will avoid having duplicated content which can get out of synch, having someone who wants to read the doc contribution guidelines have to click through to a file on GitHub is not onerous.