Document that plugins.allowRemoteAdmin option must be enabled

Also mention, that currently only the members of the administrator
group allowed to use this plugin.

Change-Id: I97e4f8fb9aba5424589441ae58cbb4575fb075a3

README.md

diff --git a/README.md b/README.md
index 6405b5c..9c7e361 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,7 @@
 # plugin-manager
 Gerrit web-based plugin manager
+
+To enable this plugin, `plugins.allowRemoteAdmin = true` is required in
+gerrit.config.
+
+Also note that the use of this plugin is restricted to Gerrit administrators.