Add a README

This makes the GitHub homepage pretty and makes it more obvious how to start contributing. Known issues with the README:

* It embeds a Travis-CI "build passing"/"build failing" badge for `flysight/flysight`, which doesn't yet exist. That can be fixed by #15 or by removing the image on line 3.
* It doesn't give helpful advice for developing on Windows.
* I saw something about NEO-7 but I don't know exactly which chip is going in, so I didn't specify.

The README [looks like this](https://github.com/willglynn/flysight) in my fork.