Add readme note about python 2 eol in readme
Add readme note about python 2 eol in readme

Now that we're getting ready to drop python 2 support for an stestr
3.0.0 release in early 2020 this commit adds a release note to make the
plans clear to end users.

masayukig/stestr,masayukig/stestr,mtreinish/stestr,mtreinish/stestr