Improve internationalization docs

Open James Ramsay requested to merge jramsay-4012-improve-internationization-docs into master

What does this MR do?

Add more information about the process of internationalization and how community members can contribute.

  • externalization (anyone)
  • translation (anyone)
  • proof reading (managed in CrowdIn)
  • release (managed in GitLab)
Edited by James Ramsay