Updated README with Z^2_N and chi^2 info

Updated README.md for v3.0.1

Merge pull request scottransom#106 from scottransom/python3

Finally merging this very long-standing set of PRs!  Thank you all, especially @matteobachetti @gijzelaerr and @paulray !  Cheers!

Updated README.md for tagging of v2.2

Update README.md

Version 2 release