Add a `CONTRIBUTING.md` file

Extract the linked remote information on contributing to a dedicated
new root file CONTRIBUTING. This is both ensuring the README remains
concise and also allows to take advantage of GitHub's usability feature
that prompts people to read a repo's guidelines before reporting issues
or opening pull requests.

[Issue(s) #2]
Remove references to "development" branches, since we aim to only have one long running branch, "master"
Merge pull request #3 from cityindex/issue2-create-issue-guidelines

Add a `CONTRIBUTING.md` file