Merge pull request OpenZeppelin#691 from frangio/fix-643

Add note about not following semver

README.md

@@ -23,10 +23,12 @@ truffle init
 
 To install the OpenZeppelin library, run the following in your Solidity project root directory:
 ```sh
-npm init
-npm install zeppelin-solidity
+npm init -y
+npm install -E zeppelin-solidity
 ```
 
+**Note that OpenZeppelin does not currently follow semantic versioning.** You may encounter breaking changes upon a minor version bump. We recommend pinning the version of OpenZeppelin you use, as done by the `-E` (`--save-exact`) option.
+
 After that, you'll get all the library's contracts in the `node_modules/zeppelin-solidity/contracts` folder. You can use the contracts in the library like so:
 
 ```js