Docs?

@astrofrog I wanted to have a quick look at what is already available in `python-reprojection`, but as far as I can see there's no docs online?
Actually the docs build doesn't seem to work at the moment:
```
$ python setup.py build_sphinx
Freezing version number to reproject/version.py
running build_sphinx
error: error in 'source_dir' option: 'docs' does not exist or is not a directory
```

I'll have a look through the open issues and what code from `gammapy` could fit in this repo on the weekend.