Update installation guide to include submodules

Author: agarwalrounak

docs/install.rst

@@ -77,10 +77,11 @@ you need these libraries.
 Get and run Read the Docs
 -------------------------
 
-Clone the repository somewhere on your disk and enter to the repository::
+Clone the repository somewhere on your disk, enter to the repository, and initialize and update the submodule::
 
     git clone https://github.com/rtfd/readthedocs.org.git
     cd readthedocs.org
+    git submodule update --init
 
 Create a virtual environment and activate it::