Document the need to copy btcd's certificate.

README.md

@@ -73,10 +73,25 @@ want btcd and btcwallet running.
 
 ### Linux/BSD/POSIX/Source
 
-- Run the following commands to start btcd and btcwallet:
+- Run the following command to start btcd:
 
 ```bash
 $ btcd --testnet -u rpcuser -P rpcpass
+```
+
+- Copy btcd's autogenerated rpc.cert to ~/.btcwallet/ so btcwallet can
+  securely communicate with btcd over a TLS-encrypted websocket.  This
+  step is only necessary for the first time starting btcwallet or if
+  btcd's autogenerated cert and key are removed and regenerated.
+
+```bash
+$ mkdir ~/.btcwallet/
+$ cp ~/.btcd/rpc.cert ~/.btcwallet/cert.pem
+```
+
+- Run the following command to start btcwallet:
+
+```bash
 $ btcwallet -u rpcuser -P rpcpass
 ```