0.20 release notes

[d2fn]

I need to write up some notes for 0.2 and cut a release

[calavera]

Some notes that might be helpful:

• Support for Mercurial.
• Support for Subversion.
• Download transitive dependencies that include a gopack.config file.
• Import stats summary.
• Dependencies list summary.
• Checksum generation for fixed dependencies (commit and tag).
• Repo structure linkage.