Updated README.md for v0.10.x
Updated README.md for v0.10.x

This commit updates the README.md file for v0.10.x. It expands the
summary of what Mutagen does and replaces most of the introductory
information with links to the documentation.

Signed-off-by: Jacob Howard <f862f167b85d41b225785c70d70808bc7337c1fe@havoc.io>

havoc-io/mutagen,havoc-io/mutagen