Add valvelet (#594)

readme.md

@@ -374,6 +374,7 @@
 	- [rfpify](https://github.com/samverschueren/rfpify) - Promisify a result-first callback-style function.
 	- [delay](https://github.com/sindresorhus/delay) - Delay a promise a specified amount of time.
 	- [promise-memoize](https://github.com/nodeca/promise-memoize) - Memoize promise-returning functions, with expire and prefetch.
+	- [valvelet](https://github.com/lpinca/valvelet) - Limit the execution rate of a promise-returning function.
 - Callbacks
 	- [each-async](https://github.com/sindresorhus/each-async) - Async concurrent iterator like forEach.
 	- [async](https://github.com/caolan/async) - Provides straight-forward, powerful functions for working with asynchronicity.