Document version tags

README.md

@@ -26,6 +26,19 @@ If there's an SSL certificate/key (`electrumx.crt`/`electrumx.key`) in the `/dat
 
 By default only the SSL port is exposed. You can expose the unencrypted TCP port with `-p 50001:50001`, although this is strongly discouraged.
 
+### Version
+
+You can also run a specific version of ElectrumX if you want.
+
+```
+docker run \
+  -v /home/username/electrumx:/data \
+  -e DAEMON_URL=http://user:pass@host:port \
+  -e COIN=Bitcoin \
+  -p 50002:50002 \
+  lukechilds/electrumx:v1.2.1
+```
+
 ## License
 
 MIT © Luke Childs