Denote the fact that the API is still unstable.

As of yesterday we were still making breaking changes to the core API of this library.  I believe we should mark it as "BETA" and explain our versioning policy clearly.  Once we commit to a 1.0, non-beta release I believe we cannot make major breaking changes to the API except as part of a major version release.  We should do that very infrequently and only when justified by substantial improvements to the library.

r? @cosn