Improve documentation / README

[yymao]

We can use a more thorough review of the current documentation (in particular, README.md on the front page), adding references to the public functions that we recently introduced in GCRCatalogs.register and their recommended usages.

Also, in lieu of a webpage that lists the public API, we can point the users to using help(GCRCatalogs.register) to see the list of public functions.

Other suggestions to improve the documentation / README are also welcome.