Update README per DOC-1501

1. Remove any "what this affects" sections, except where particularly warranted.
2. Make sure that for each parameter, where applicable, there is a data type and a default value.
3. Make sure that for each parameter that's applicable, there is a note if the parameter is optional.
4. Make sure the links in the README work and are accurate.
5. Update the link in the Contributing section to point here: https://docs.puppetlabs.com/forge/contributing.html
6. General copyediting.