Roadmap for Python version compatibility

[akaihola]

In issue #4 and PR #12, hardlinkpy's compatibility with different Python versions has been discussed.

@chadnetzer currently aims to support Python all the way back to version 2.3 due to RHEL 5 and other distributions. On the other hand, compatibility with the 3.x series is just being finished.

The policy for Python version compatibility could be explicitly stated in the README file.

A timeline for removing support for old Python versions could also be planned. It could be synchronized e.g. with end-of-life for RHEL releases and Python versions shipped with RHEL.