Update README per DOC-1503

Update README per DOC-1503

1. Make sure that for each parameter, where applicable, there is a default value.
2. Make sure that for each parameter that's applicable, there is a note if the parameter is optional.
3. Make sure the links in the README work and are accurate.
4. Update the link in the Contributing section to point here: https://docs.puppetlabs.com/forge/contributing.html
5. General copyediting.

* Add a ToC
* Reorganize Usage & Reference sections
* Standardize capitalization of "Apt" (as opposed to "apt", the module)
* Standardize Development section

Apply edits from @jbondpdx

* Re-emphasize the warning about short key names
* Restore the “What apt affects” section
* Re-correct capitalization of “Apt” (where it had reverted to “APT”)
* Clarify OS compatibility in Limitations and elsewhere
* Various edits for clarity