Tidy up top-level documentation

Grouped a bunch of small changes into one.

Removed the list of contributors. This is included in the GitHub UX. And from experience, it’s a PITA to maintain anyway.

Created a `CONTRIBUTION.md` doc, which will show up when people are creating tickets. This makes the license stuff explicit, as well as serving as useful documentation.

Moved the mention of the TODO wiki page to the contribution guide.

Want to remove “| (• ◡•)| (❍ᴥ❍ʋ)” but can’t bring myself to do it. Is this an adventure time ASCII art?

Altered license so the Apache License 2.0 license text is quoted inline. This will allow us to add more licenses to this file if and when we vendor in 3rd party libraries.

Created the `NOTICE.md` file to track copyright statements, where we have to give them. At the moment, this only lists one, which asserts our collective copyright over the work. This is all that’s needed right now. If we add 3rd party libraries, we may need to add more things to this file.

Here’s an example of a plain text LICENSE and NOTICE file with lots of stuff in:

https://github.com/apache/couchdb/blob/master/NOTICE

https://github.com/apache/couchdb/blob/master/LICENSE

David, this means that you are no longer credited in the copyright. If you’d like to have your name listed somewhere as the primary maintainer, I’d probably put that as a note in the README.md, but it should be obvious enough again from the location of the repo.

In general, I have found that the further away you keep individual names from a project, the more likely it is that you can attract contributors because there’s less of a perceived feeling of “us and them” (i.e. in-group and out-group) which can be off-putting for a newbie.

This will closes #71.