Improve installation and contribution docs

Make it more clear what Tox is for and how to do the Pip based package install and avoid the typical Black beta release error. Also show that this package is only Python >= 3.5.

As a result of https://lab.libreho.st/decentral1se/librehosters-cli/issues/1.

Signed-off-by: Luke Murphy lukewm@riseup.net