The following sites will be helpful in determining what rights you want to maintain, how to use an addendum, and offer templates to copy:
This is from SPARC (Scholarly Publishing & Academic Resources Coalition) with a basic template that can be downloaded.
This is from the Science Commons and provides a dynamic website that allows you to build an addendum online using their template.
This MIT amendment is well done and gives you good instructions.
This is a very good site from OhioLink that does a comparison of various author addenda.
For an overview of what individual publishers are doing vis a vis accepting amendments to their publication agreements:
Sherpa Romeo is a non-profit organization which tracks scholarly journal publishers.