Add missing CONTRIBUTING.md to source tarball

Fixes MANIFEST.in to include missing CONTRIBUTING.md into source
code tarballs.

Change-Id: I81876adad14e0f1b2211d39ec6471e35f5464120
Fix swift-ring-builder's list_parts command.

I broke it in 7548cb9 when a ring's replica count changed from an int
to a float.

Change-Id: I49fa7e2961a09daad0c96e19cf1e39fec248d998
Merge "Fix swift-ring-builder's list_parts command."
Merge "Add missing CONTRIBUTING.md to source tarball"