[gtk] (6 commits) ...Merge branch 'ebassi/howto-docs' into 'master'

[Emmanuele Bassi <ebassi src gnome org>]

Summary of changes:

  25a8fa7... docs: Turn coding style into a markdown document (*)
  6b6c76e... docs: Remove last two steps from the release howto (*)
  e30c974... Add myself to more sections (*)
  c5485cd... Add the beginnings of a docs contribution guide (*)
  347cb94... docs: Link the documentation guidelines (*)
  85af410... Merge branch 'ebassi/howto-docs' into 'master'

(*) This commit already existed in another branch; no separate mail sent