Add instructions on initializing submodules

BUILDING.md

@@ -3,6 +3,12 @@
 
 ## Setting up Your Environment
 
+If you cloned the repository you will need to set up the git submodules (if you downloaded and extracted a release, you can skip this step):
+
+.. code-block:: bash
+
+    > git submodule init
+    > git submodule update
 
 VTR requires several system packages.  From the top-level directory, run the following script to install the required packages on a modern Debian or Ubuntu system:
 

doc/src/quickstart/index.rst

@@ -17,6 +17,13 @@ The first step is to `download VTR <https://verilogtorouting.org/download/>`_ an
 
 Environment Setup
 -----------------
+If you cloned the repository you will need to set up the git submodules (if you downloaded and extracted a release, you can skip this step):
+
+.. code-block:: bash
+
+    > git submodule init
+    > git submodule update
+    
 VTR requires several system packages and Python packages to build and run the flow.  You can install the required system packages using the following command (this works on Ubuntu 18.04, 20.04 and 22.04, but you may require different packages on other Linux distributions). Our CI testing is on Ubuntu 22.04, so that is the best tested platform and recommended for development.
 
 .. code-block:: bash