This commit adds details for a developer to contribute to TCE. Namely
building and installing the CLI and plugins.

This commit also updates the Makefile with the following changes.

* Help text (make help) is only presented for commands that are meant to
  be run locally by developers. All other commands are considered
  "private".
* Targets that install plugins or the tanzu CLI now tell the user where
  they were installed (assuming successful completion)