Add git paragraph to installation section (OpenZeppelin#3197)

Author: frangio

README.md

@@ -22,6 +22,8 @@ $ npm install @openzeppelin/contracts
 
 OpenZeppelin Contracts features a [stable API](https://docs.openzeppelin.com/contracts/releases-stability#api-stability), which means your contracts won't break unexpectedly when upgrading to a newer minor version.
 
+An alternative to npm is to use the GitHub repository `openzeppelin/openzeppelin-contracts` to retrieve the contracts. When doing this, make sure to specify the tag for a release such as `v4.5.0`, instead of using the `master` branch.
+
 ### Usage
 
 Once installed, you can use the contracts in the library by importing them: