Correct and improve `docs/` README.md.
Correct and improve `docs/` README.md.

These instructions for the docs portion of Apollo Client are out of date and
no longer this complicated as there are no submodules in this repository
anymore.

This also introduces some additional helpful details to hopefully aid anyone
who is interested in helping docs improvements (which are always welcome!)

cc @jbaxleyiii (Fyi)

apollographql/apollo-client,apollostack/apollo-client,apollostack/apollo-client,apollostack/apollo-client,apollographql/apollo-client