Update CONTRIBUTING.md

update CONTRIBUTING.md with more information
Adding icon to Atom toolbox choice on dropdown of recent changes/history pages.
Fix for bug #137, provide a parent container for mw-content-text div.
Merge pull request #131 from Hutchy68/develop

Update CONTRIBUTING.md
Merge pull request #133 from Hutchy68/fix/atom-icon

[bug][issue #132] Adding icon to Atom toolbox choice
Merge pull request #138 from Hutchy68/fix/mw-content-div

[bug][issue #37] Provide a parent container for mw-content-text div.
Merge branch 'develop' into feature/navbar-icon