remote-wallet: add README

Add a README to remote-wallet which explains why this library exists and
where to get ledger udev rules from.

remote-wallet/README.md

@@ -0,0 +1,10 @@
+Solana Remote Wallet
+===
+
+Library for interacting with "remote" wallets, meaning any wallet where the private key bytes are not directly available,
+such as Ledger devices.
+
+## Ledger udev-rules
+
+In order to use a Ledger device on Linux machines, users must apply certain udev rules. These are available at the
+[udev-rules repository](https://github.com/LedgerHQ/udev-rules) maintained by the Ledger team.