add info to readme file to point out other doc files.

[jonvnadelberg]

Description

The repositories for Sigstore have a readme.md file that describes some of what is going on with the associated modules in the docs folder. In addition, there are some repositories that in addition have doc files that contain further information. This issue is for the editing of the readme files to contain short descriptions of the other documentation files and a reminder that docs must be updated to keep in sync with any code changes.

The following repositories will have changes:

Cosign
Fulcio
Rekor

Each repository will be forked and have a branch named jvn-257. The changes will be in that branch for each repository.