Create Readme.md file for having release notes (#6023)

Create Readme.md file for having release notes

For Dynamic links iOS SDK, we don't have a place to communicate the latest changes in the release to 3p developers. 

Adding a readme.md file to inform developers on the recent changes in SDK release and other info that developers should be aware of.

FirebaseDynamicLinks/CHANGELOG.md

@@ -4,6 +4,8 @@
 - [fixed] Reduce frequency of iOS14 pasteboard notifications by only reading from it when it contains URL(s). (#5905)
 - [changed] Functionally neutral updated import references for dependencies. (#5824)
 
+Refer to the [README.md](https://github.com/firebase/firebase-ios-sdk/blob/master/FirebaseDynamicLinks/README.md) for more details about this release.
+
 # v4.1.0
 - [fixed] Fixing unwanted pending dynamic links checks on subsequent app restarts. (#5665)
 

FirebaseDynamicLinks/README.md

@@ -0,0 +1,35 @@
+# Firebase Dynamic Links SDK for iOS
+
+Firebase Dynamic Links are universal deep links that persist across app installs.
+For more info, see the [Firebase website](https://firebase.google.com/products/dynamic-links).
+
+Please visit [our developer site](https://firebase.google.com/docs/dynamic-links/) for integration
+instructions, documentations, support information, and terms of service.
+
+## Managing the Pasteboard
+
+Firebase Dynamic Links 4.2.0 and higher use a plist property
+`FirebaseDeepLinkPasteboardRetrievalEnabled` that a developer can set to enable/disable the use of
+iOS pasteboard by the SDK.
+
+FDL SDK uses the pasteboard for deep-linking post app install (to enable deferred deep-linking,
+where the link is copied on the
+[app preview page](https://firebase.google.com/docs/dynamic-links/link-previews#app_preview_pages))
+and app install attribution; otherwise, FDL does not use the pasteboard for anything else.
+
+Disabling pasteboard access affects the app in the following ways:
+* Deferred deep-linking will not work as reliably.  At best, your app receives
+[weak matches](https://firebase.google.com/docs/reference/unity/namespace/firebase/dynamic-links#linkmatchstrength)
+for deep-links.
+* App install attribution stats will be less accurate (potentially undercounting app installs).
+
+Enabling pasteboard access affects the app in the following ways:
+* On iOS 14, will show a system alert notifying that your app accessed the content in the
+pasteboard. This should happen one-time after installation of the app.
+* Deferred deep-linking will work as designed.  At best, your app receives a
+[perfect match](https://firebase.google.com/docs/reference/unity/namespace/firebase/dynamic-links#linkmatchstrength)
+for deep-links.
+* SDK will be able to more reliably attribute installation stats for links.
+
+For more information, check out the
+[iOS documentation](https://firebase.google.com/docs/dynamic-links/ios/receive).