doc: mention git submodule update

I at least once failed to build gitiles since I did not initialize git
submodule. Add a mention about it.

Change-Id: I1c60a9455cd5c2ab64599b8b3c6644881914bc67

Documentation/developer-guide.md

diff --git a/Documentation/developer-guide.md b/Documentation/developer-guide.md
index 70b050e..6f809fd 100644
--- a/Documentation/developer-guide.md
+++ b/Documentation/developer-guide.md
@@ -21,6 +21,12 @@
 $ go install github.com/bazelbuild/bazelisk@latest
 $ export PATH=$PATH:$(go env GOPATH)/bin
 ```
+
+Make sure to initialize and update the git submodules:
+```
+git submodule update --init
+```
+
 You are now ready to build and test.
 
 ```