README.md: Add instructions for testing local changes.

BUG=None
TEST=None

Change-Id: I17341e36056b568247c49ce2947edeb8419b2833
diff --git a/README.md b/README.md
index ddd1ccb..793e520 100644
--- a/README.md
+++ b/README.md
@@ -14,3 +14,13 @@
   https://android.googlesource.com/platform/bbuildbot_config \
   bbuildbot
 ```
+
+To perform a test build with local changes:
+
+```sh
+bin/cbuildbot --nobootstrap --noreexec \
+  --buildroot ~/tmp/test_buildroot \
+  --buildbot --debug --config_repo  \
+  https://android.googlesource.com/platform/bbuildbot_config \
+  bbuildbot
+```