update README

Author: jprjr

README.md

@@ -24,6 +24,7 @@ This requires your web server to have an active instance of
  * New `resty.exec.socket` module for using a duplex connection
  * `resty.exec` no longer uses the `bufsize` argument
  * `resty.exec` now accepts a `timeout` argument, specify in milliseconds, defaults to 60s
+ * This is a major revision, please test thoroughly before upgrading!
 * No changelog before `2.0.0`
 
 ## Installation