docs: add note on cloning recursively (#95)

Signed-off-by: JP Simard <jp@jpsim.com>

mobile/docs/root/start/building/building.rst

@@ -9,6 +9,13 @@ In order to compile the artifacts used by the Envoy Mobile library,
 your system must also meet
 `these requirements for building Envoy <https://github.com/envoyproxy/envoy/tree/master/bazel>`_.
 
+Ensure that the ``envoy`` **submodule** is initialized when cloning by using ``--recursive``:
+
+``git clone https://github.com/lyft/envoy-mobile.git --recursive``
+
+If the repo was not initially cloned recursively, initialize the Envoy
+submodule with ``git submodule update --init``.
+
 --------------------
 Android requirements
 --------------------