Improve docs in docs.rs and remove repetition from README.md

[andrewdavidmackenzie]

CHeck the list of methods implemented as listed in readme. If there are any important ones missing, then create issues for them.

Improve the docs in docs.rs that a user of the lib would consult, to make api methods available more obvious and easier to understand.

Put some of the readme contents as crate level docs so readers know what they are getting when they go direct to docs.rs

[andrewdavidmackenzie]

Closed by https://github.com/andrewdavidmackenzie/libproc-rs/pull/124