explicitly say how to launch the app for iOS (#2401)

*Replace this paragraph with a description of what this PR is changing
or adding, and why. Consider including before/after screenshots.*

*List which issues are fixed by this PR. For larger changes, raising an
issue first helps
reduce redundant work.*

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

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add_to_app/fullscreen/README.md

@@ -38,6 +38,7 @@ you're building for both iOS and Android, with both toolchains installed):
   cd ../ios_fullscreen
   pod install
   open IOSFullScreen.xcworkspace
+  # Then, click "Run" in Xcode to launch the app into your Simulator or device
 ```
 
 ## Requirements