Add Missing README.md Files

[aszepieniec]

Apparently it's customary to add a README.md file per crate as opposed to per repository as is the case now. Here is the view of neptune-core-cli on crates.io:

Neptune-dashboard has a similar view, but the existing README.md does show up dutifully for neptune-cash.

[aszepieniec]

On a somewhat related note, upon publishing crate neptune-rpc-macros v0.5.0 I get this error message:

error: neptune-rpc-macros is missing the following fields:
  description
  license || license-file
  documentation || homepage || repository

So let's also add the missing data there.