Add README.md to source dist.

As per this commit to resolve a similar issue
https://github.com/saily/python-memcached/commit/829fa09718ab75a6988eb7ef5857fd50bf942749
https://github.com/linsomniac/python-memcached/issues/5
Merge branch 'adamlwgriffiths/master' into master

This is for the improved MANIFEST.in file.

Conflicts:
	MANIFEST.in

fixes #1 and fixes #2