Add `CONTRIBUTING.md`
Add `CONTRIBUTING.md`

Providing the `CONTRIBUTING.md` file alows us to take advantage
of GitHub's UI and prompt contributors to read the repository's
guidelines before creating an issue or opening a pull request:
https://github.com/blog/1184-contributing-guidelines.

The content in the `CONTRIBUTING.md` file is based on @necolas's
Issue Guidelines: https://github.com/necolas/issue-guidelines.

Gillespie59/server-configs-apache,atyenoria/server-configs-apache,sengkoil/server-configs-apache,h5bp/server-configs-apache,blackmoral/server-configs-apache,walterebert/server-configs-apache,StephanieMak/server-configs-apache,voku/server-configs-apache,voku/server-configs-apache,johnnyicarus/server-configs-apache,StephanieMak/server-configs-apache,Gillespie59/server-configs-apache,atyenoria/server-configs-apache,walterebert/server-configs-apache,johnnyicarus/server-configs-apache,blackmoral/server-configs-apache,sengkoil/server-configs-apache