This replaces the Review Bot README with a ReST-based version that

summarizes the important parts of Review Bot and defers to the manual

for the rest. It helps keep the important pieces of information in one

place (the manual) and gives viewers looking on GitHub a better idea of

where to start.
Worker and extension-specific READMEs have also been added. These will

be added to the packages in an upcoming change, so that they'll be

included in the source distribution and shown on PyPI.

Viewed all the rendered READMEs and tested links.