Document new command in readme.md

Author: itaisteinherz

README.md

@@ -20,6 +20,16 @@ All functions are compiled with webpack using the Babel Loader, so you can use m
 
 Before developing, clone the repository and run `yarn` from the root of the repo to install all dependencies.
 
+### Start both servers concurrently
+
+From inside the project folder, run:
+
+```
+yarn start
+```
+
+This will run both `yarn start:lambda` and `yarn start:app` at the same time (more information about the commands can be found below).
+
 ### Start each server individually
 
 **Run the functions dev server**