Add note to use init when upgrading

Due to the migration performed in Ic5c4e8188, it is needed to run
Init when performing an ugprade for the Checks plugin.

Change-Id: I4b09f443f34b2313244042d4e211cc2d8728b8d9
diff --git a/README.md b/README.md
index 4a61e1e..41f6dfd 100644
--- a/README.md
+++ b/README.md
@@ -8,3 +8,8 @@
 After `bower install`, running `polymer test -l chrome` will run all tests in Chrome, and running `polymer serve`
 and navigating to http://127.0.0.1:8081/components/checks/gr-checks/gr-checks-view_test.html allows for manual debugging.
 
+When upgrading the plugin, please use init:
+
+    java -jar gerrit.war init -d site_path
+
+More details about "init" in https://gerrit-review.googlesource.com/Documentation/pgm-init.html