README: update with description

Author: Roasbeef

README.md

@@ -1,6 +1,14 @@
+## Lightning Desktop App
+
+This repo houses a cross-platform Lightning desktop app powered by
+[`lnd`](https://github.com/lightningnetwork/lnd/). The application is under
+active development and currently only operates on the Bitcoin testnet chain.
+
+<img src="screenshot.png">
 
 ### Developing Locally
 
+
 First start by pulling down the git repo:
 ```
 git clone https://github.com/lightninglabs/lightning-app.git