Add version requirement according API requirement (#15)

diff --git a/README.md b/README.md
index 16b75dd..5a46aba 100644
--- a/README.md
+++ b/README.md
@@ -12,6 +12,8 @@
 
 ## Usage
 
+Kong 2.2+ required.
+
 1. Install the plugin in Kong:
 
 Install kong-plugin-skywalking using `luarocks`: