Add nodejs-legacy instruction to README.md.

Fixes: #26.

README.md

@@ -17,12 +17,21 @@ $ git clone https://github.com/zulip/zulip-electron
 $ cd zulip-electron
 ```
 
+Use your package manager to install `npm`.
+
+If you're on Debian or Ubuntu, you'll also need to install
+`nodejs-legacy`:
+
+```sh
+$ sudo apt-get install npm nodejs-legacy
+```
+
 Install project dependencies:
 
 ```sh
 $ npm install
 ```
-Start the App
+Start the app:
 
 ```sh
 $ npm start