Add README.md for Sample app. (#20)

* Add README.md for Sample app.

* Update README.md

Author: petea

Sample/README.md

@@ -0,0 +1,29 @@
+# Google Sign-In Sample App
+
+## CocoaPods
+
+1. In the Sample folder, run the following [CocoaPods](https://cocoapods.org) command
+   to install the required dependencies:
+
+```
+pod install
+```
+
+2. Open the generated workspace:
+
+```
+open SignInSampleForPod.xcworkspace
+```
+
+3. Run the 'SignInSample' target.
+
+## Swift Pacakage Manager
+
+1. In the Sample folder, open the
+   [Swift Package Manager](https://swift.org/package-manager/) project:
+
+```
+open SignInSample.xcodeproj
+```
+
+2. Run the 'SignInSample' target.