Added the CLA

Since this is a Google owned open-source repo, contributors will have
to fill out a Contributor License Agreement.

Author: puf

README.md

@@ -161,7 +161,7 @@ FirebaseDataSource acts as a generic data source by providing common information
 - From your macbook that already has been granted permissions to Firebase Cocoapods, do `pod trunk push`
 - Update [firebase-versions](https://github.com/firebase/firebase-clients/blob/master/versions/firebase-versions.json) with the changelog for this release.
 
-## Contributing
+## Local Setup
 
 If you'd like to contribute to FirebaseUI for iOS, you'll need to run the
 following commands to get your environment set up:
@@ -171,3 +171,34 @@ $ git clone https://github.com/firebase/FirebaseUI-iOS.git
 $ cd FirebaseUI-iOS
 $ ./setup.sh
 ```
+
+## Contributor License Agreements
+
+We'd love to accept your sample apps and patches! Before we can take them, we
+have to jump a couple of legal hurdles.
+
+Please fill out either the individual or corporate Contributor License Agreement
+(CLA).
+
+  * If you are an individual writing original source code and you're sure you
+    own the intellectual property, then you'll need to sign an [individual CLA]
+    (https://developers.google.com/open-source/cla/individual).
+  * If you work for a company that wants to allow you to contribute your work,
+    then you'll need to sign a [corporate CLA]
+    (https://developers.google.com/open-source/cla/corporate).
+
+Follow either of the two links above to access the appropriate CLA and
+instructions for how to sign and return it. Once we receive it, we'll be able to
+accept your pull requests.
+
+## Contributing A Patch
+
+1. Submit an issue describing your proposed change to the repo in question.
+1. The repo owner will respond to your issue promptly.
+1. If your proposed change is accepted, and you haven't already done so, sign a
+   Contributor License Agreement (see details above).
+1. Fork the desired repo, develop and test your code changes.
+1. Ensure that your code adheres to the existing style of the library to which
+   you are contributing.
+1. Ensure that your code has an appropriate set of unit tests which all pass.
+1. Submit a pull request.