Add metaphore (cache slam defense library). (#703)

README.md

 * [CacheTool](https://github.com/gordalina/cachetool) - A tool to clear APC/opcode caches from the command line.
 * [Cake Cache](https://github.com/cakephp/cache) - A caching library (CP).
 * [Doctrine Cache](https://github.com/doctrine/cache) - A caching library.
+* [Metaphore](https://github.com/sobstel/metaphore) - Cache slam defense using a semaphore to prevent dogpile effect.
 * [Stash](https://github.com/tedious/Stash) - Another library for caching.
 * [Zend Cache](https://github.com/zendframework/zend-cache) - Another caching library (ZF2).