#27 Improved contribution information.

 - CONTRIBUTING file improvements:
   - Improved list of must useful links.
   - Refer to contributing documentation.
   - Don't refer contributors to merge requests of official Git repo.
   - Explain commit message format.
 - Contributing documentation improvements:
   - Explain how to create an Eclipse Foundation account.
   - Improved and extended information on contributing via GitLab.
   - Added information on how to sync with official Git repo.
   - Added information on how committers/contributors can collaborate.