Update README.md

I clarified where the "npm install zeppelin-solidity" command should happen. I got hung up on it initially and I know others have as well.

README.md

@@ -21,7 +21,7 @@ mkdir myproject && cd myproject
 truffle init
 ```
 
-To install the OpenZeppelin library, run:
+To install the OpenZeppelin library, run the following in your Solidity project root directory:
 ```sh
 npm init
 npm install zeppelin-solidity