[alias-server] Clean up README as prep for documenting API endpoints

Summary:
T3060

Clean up legacy / unused TODO info from README and replace with stub API documentation

Will be expanded as new endpoints are added

Note: the '/aliases' endpoint will either be deprecated or paginated, in another diff

Test Plan: Verify only the README is changed, no typos

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14128