Add version field to GetInfo RPC endpoint

This is inspired by the Bitcoin RPC `getnetworkinfo` endpoint which also
includes version information:
https://bitcoin.org/en/developer-reference#getnetworkinfo

lnrpc/README.md

@@ -160,4 +160,4 @@ $ git reset --hard f2862b476edcef83412c7af8687c9cd8e4097c0f
 $ go install ./protoc-gen-grpc-gateway ./protoc-gen-swagger
 ```
 
-4. Run `gen_protos.sh` to generate new protobuf definitions.
+4. Run [`gen_protos.sh`](https://github.com/lightningnetwork/lnd/blob/master/lnrpc/gen_protos.sh) to generate new protobuf definitions.