Introduce the Branch Sandbox Program

Add instructions to README.md on how to join the Branch Sandbox Program.

Author: agerstner-branch

README.md

@@ -1,3 +1,13 @@
-## Branch SDK Documentation for iOS
+# Branch SDK Documentation for iOS
+
+The Branch iOS SDK for deep linking and attribution.
+
+> ## Branch Sandbox Program
+>
+> As part of this [sandbox program](https://help.branch.io/developers-hub/docs/branch-sandbox-program), you will be able to partner directly with Branch’s Product and Engineering team for exclusive access to test drive our innovative products before the market and proactively exchange valuable feedback. With your support, we will create the winning measurement and linking solutions of the future together.
+> 
+> **If you would like to join, please reach out to us today at [[email protected]](mailto:[email protected] "mailto:[email protected]") or by filling out this [form](https://forms.gle/i8Fjxhjh7oY6fDfUA).**
+
+Branch helps mobile apps grow with deep links / deeplinks that power paid acquisition and re-engagement campaigns, referral programs, content sharing, deep linked emails, smart banners, custom user onboarding, and more.
 
 View [Branch's SDK documentation for iOS](https://help.branch.io/developers-hub/docs/ios-sdk-overview)