Also mention 0.13

0.13 release announcement mentions this plugin, in this version:

https://groups.google.com/d/msg/simple-build-tool/0AGST5qPbzw/CrN1sJ6ut-AJ

Hence, the README should be updated as well.
Merge pull request #38 from Blaisorblade/patch-1

README: mention SBT 0.13