docs: README is stale — rm and ln are implemented

[lionello]

The README no longer matches the code:

• Command list says rm — delete a file from OneDrive (not implemented). It is implemented (rm, rmItem, del), handles the 204-No-Content response, and recurses into folders.
• TODO list still contains "Implement rm".
• TODO list still contains "Ability to get the link for a file" — that's ln (see Question about "Ability to get the link for a file" #12).

Fixes: drop the "(not implemented)" note, move both entries to DONE, and close #12.

While in there, the TODO list is worth re-syncing against the sub-issues on this tracker so there's one source of truth.