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.

----
See https://github.com/cityindex/remote-development-boilerplate/issues/2#issuecomment-15006486 for the background - the one thing I'm still not entirely happy with (despite defusing the original headings already) is section [Using the issue tracker](https://github.com/cityindex/remote-development-boilerplate/blob/4ce809537b01600ecbf91410e8b858e57eb94368/CONTRIBUTING.md#using-the-issue-tracker), insofar the two _**do not ...**_ instructions do not seem to apply for all but high volume projects; regardless, I think this is good enough as a starting point and can be adjusted over time as we see fit.