Create walkthrough's / readme's for Credential Registry

[science]

• Create and publish an envelope
  • Examples for credential and envelope
  • Integration with envelope
  • Signing with JWT
  • Publishing
    • What happens when a document has an error/fails validation
• Retrieve an envelope
• Other docs / info that @andersoncardoso thinks will improve the developer experience

[andersoncardoso]

https://github.com/learningtapestry/learningregistry/blob/master/docs/credential_registry_walkthrough.md

We will improve on this iteratively.