README: document solo stamping

README.md

@@ -105,6 +105,19 @@ commitment operations and attestations in it:
         append 647b90ea1b270a97
         verify PendingAttestation('https://bob.btc.calendar.opentimestamps.org')
 
+### Timestamping without a calendar service with your own wallet
+
+It's possible to create a timestamp using your own Bitcoin Core wallet. This
+is generally not necessary, can harm your privacy and is somewhat risky because
+the code is not well tested.
+
+Use the `--btc-wallet` argument after `stamp` command to use this feature,
+optionally specifying a fee rate in sat/vbyte with `--fee-rate`. It is
+possible to RBF the resulting transaction and then resume the process using
+the `--txid` and `--nonce` arguments.
+
+See `ots stamp --help` for more info.
+
 ### Timestamping and Verifying PGP Signed Git Commits
 
 See `doc/git-integration.md`