Please update README with current best practices for installing

[pboushy]

Looks like python-jss was updated for Python3, and has a new version (2.1.0) that was released.
But the instructions for installing don't work at all in Python3 - pip install doesn't work.
In addition, searching through closed issues, there's a comment where you mention that 1.5.0 is old and shouldn't be used, but releases still shows 1.5.0 is the current versions.

Would appreciate it if all this info could be updated so that people know where to go to get the actual latest.