Update readme.md to suggest Metadata Cache adapter under 3rd party adapters.

This metadata cache adapter can be used to improve performance.

Author: jgivoni

readme.md

@@ -47,6 +47,8 @@ for which ever storage is right for you.
 * **[ReplicateAdapter](https://github.com/ajgarlag/flysystem-replicate)**
 * **[Uploadcare](https://github.com/vormkracht10/flysystem-uploadcare)**
 * **[Useful adapters (FallbackAdapter, LogAdapter, ReadWriteAdapter, RetryAdapter)](https://github.com/ElGigi/FlysystemUsefulAdapters)**
+* **[Metadata Cache](https://github.com/jgivoni/flysystem-cache-adapter)**
+
 
 You can always [create an adapter](https://flysystem.thephpleague.com/docs/advanced/creating-an-adapter/) yourself.