Update README to reflect end of Appsody development

[garypicher]

Checklist:

• [x] Read the Code of Conduct and Contributing Guidelines.

• [x] Followed the commit message guidelines.

• [x] Stack adheres to Appsody stack structure.

Modifying an existing stack:

n/a - not modifying an existing stack

Contributing a new stack:

n/a - not contributing a new stack

[CLAassistant]

All committers have signed the CLA.