Mention the `envsetup.sh` script in ART Module building instructions.

Test: Render `build/README.md` as HTML and check the output manually
Change-Id: I394525f70df55096ab1167fb37ee3bca14ff29ee
diff --git a/build/README.md b/build/README.md
index 2f9eef5..2c84130 100644
--- a/build/README.md
+++ b/build/README.md
@@ -42,6 +42,7 @@
     unbundled module build:
 
     ```
+    source build/envsetup.sh
     banchan com.android.art <arch>
     ```