Update README.md in master branch to reflect the commit rollback or any other changes relevant to the current status.

[knottyanyon]

Package built with bazel from master branch shows up as 0.7.3 in pip. Lack of details in changelists isn't very helpful to understand whether the lack of support for cirq >= 1.0.0 (after the rollback) to whether there are any significant difference between main and 0.7.2 is very confusing to analyze any breaking changes in personal projects.

suggestion:

• mention the current semantic version number with dev, alpha etc in the main README to distinguish it from the tagged current release
• adding a note temporarily in README mentioning the commit rollback from months ago if possible