Add offline signing tutorial

doc/README.md

@@ -79,6 +79,7 @@ The Bitcoin repo's [root README](/README.md) contains relevant information on th
 - [Init Scripts (systemd/upstart/openrc)](init.md)
 - [Managing Wallets](managing-wallets.md)
 - [Multisig Tutorial](multisig-tutorial.md)
+- [Offline Signing Tutorial](offline-signing-tutorial.md)
 - [P2P bad ports definition and list](p2p-bad-ports.md)
 - [PSBT support](psbt.md)
 - [Reduce Memory](reduce-memory.md)