Bazel: Document prerequisite for eclipse project generation

Change-Id: Ida285055a270a3d18eb1b7ea04ae3dc869ae0139
diff --git a/src/main/resources/Documentation/build.md b/src/main/resources/Documentation/build.md
index 13f24bc..81182de 100644
--- a/src/main/resources/Documentation/build.md
+++ b/src/main/resources/Documentation/build.md
@@ -47,7 +47,9 @@
   bazel test --test_tag_filters=@PLUGIN@ //...
 ````
 
-This project can be imported into the Eclipse IDE:
+This project can be imported into the Eclipse IDE.
+Add the plugin name to the `CUSTOM_PLUGINS` set in
+Gerrit core in `tools/bzl/plugins.bzl`, and execute:
 
 ```
   ./tools/eclipse/project.py