Document installing the develop version of esbonio

[laund]

I just ran into #393 and wanted to still work on my project without waiting for a update. Took me a while to figure out the correct pip package path so i could install the newest version with the fix:

pip uninstall esbonio
pip install "git+https://github.com/swyddfa/esbonio#egg=esbonio&subdirectory=lib/esbonio"

Would be nice if that could be mentioned somewhere in the docs.

[Alex Carney]

Thanks for the suggestion - I didn't realise you could do this!