Merge pull request #603 from oziks/update-readme-existing-plugins-sec…

…tion

Update existing plugins section of readme.

Readme.md

@@ -68,6 +68,7 @@ request
 Existing plugins:
  * [superagent-no-cache](https://github.com/johntron/superagent-no-cache) - prevents caching by including Cache-Control header
  * [superagent-prefix](https://github.com/johntron/superagent-prefix) - prefixes absolute URLs (useful in test environment)
+ * [superagent-mock](https://github.com/M6Web/superagent-mock) - simulate HTTP calls by returning data fixtures based on the requested URL
 
 Please prefix your plugin with `superagent-*` so that it can easily be found by others.