readme: install the pre version
readme: install the pre version

The latest released version is a pre-released version. It's confusing to install the latest release pip version and the docs seeming out of sync.
wbrefvem/github3.py,itsmemattchung/github3.py,jim-minter/github3.py,icio/github3.py,christophelec/github3.py,ueg1990/github3.py,degustaf/github3.py,sigmavirus24/github3.py,krxsky/github3.py,agamdua/github3.py,balloob/github3.py,h4ck3rm1k3/github3.py