Simplify install instructions

[timbmg]

The library can be directly installed from github with pip. No need to clone it first.

[kermitt2]

Hello! I've completed a bit the install documentation, as one might prefer to use the latest PyPI version, or to have the complete source (useful to have config.json or example.py on hand given that they are referenced in the readme). So the user can choose.

[lfoppiano]

I though that for the installation it was equivalent. Personally I either use the client from the source, or install it as dependency. Never do both 😅

If I may 😅, I would then propose to install with pip -e . rather than using python setup.py install, as it's more standardized and will work with other project descriptors (e.g. pyproject.toml, as proposed in PR #77).