Create roadmap for project

[science]

Include sections for "core," "learning registry," and "credential registry" where there is active work or plans for each.

Probably just create a "ROADMAP.md" file for this, and include a link to it on the README.md file.

[andersoncardoso]

Done => ROADMAP

• https://github.com/learningtapestry/metadataregistry/commit/4ec53b2ca3706685b9f3822444b79744b41c9090
• https://github.com/learningtapestry/metadataregistry/commit/5d887283a207dea64921966d0a6241bbdb0ef536