Add info about contributing to docs to CONTRIBUTING.md

Encourage more people to contribute to Rails docs by mentioning it in the contribution guide.
Merge pull request #10867 from iHiD/patch-1

Add info about contributing to docs to CONTRIBUTING.md
clearing autoloaded constants triggers routes reloading [Fixes #10685]

Conflicts:
	railties/test/application/loading_test.rb
registers recent patch in the railties CHANGELOG