Error Prone: Enable checks for core plugins

Also document how to enable the checks for custom added plugins.

Change-Id: Ifde569cfd07820de0beaf213e0b69777063b7c64

tools/BUILD

diff --git a/tools/BUILD b/tools/BUILD
index 2d0cbce..09adf16 100644
--- a/tools/BUILD
+++ b/tools/BUILD
@@ -100,5 +100,16 @@
     packages = [
         "//java/...",
         "//javatests/...",
+        "//plugins/codemirror-editor/...",
+        "//plugins/commit-message-length-validator/...",
+        "//plugins/delete-project/...",
+        "//plugins/download-commands/...",
+        "//plugins/gitiles/...",
+        "//plugins/hooks/...",
+        "//plugins/plugin-manager/...",
+        "//plugins/replication/...",
+        "//plugins/reviewnotes/...",
+        "//plugins/singleusergroup/...",
+        "//plugins/webhooks/...",
     ],
 )