Add API stability doc link. (#1672)

* Add API stability doc link.

* Update README.md

Co-Authored-By: nventuro <nicolas.venturo@gmail.com>

README.md

@@ -12,6 +12,8 @@
 npm install openzeppelin-solidity
 ```
 
+OpenZeppelin features a stable API, which means your contracts won't break unexpectedly when upgrading to a newer minor version. You can read ṫhe details in our [API Stability](https://forum.zeppelin.solutions/t/api-stability/138) document.
+
 ## Usage
 
 To write your custom contracts, import ours and extend them through inheritance.