Add fastlane to default .gitignore

Summary:
Taken from the [github/gitignore repo](https://github.com/github/gitignore/blob/master/Objective-C.gitignore)
Closes https://github.com/facebook/react-native/pull/11113

Differential Revision: D4231705

fbshipit-source-id: 3d768b6b73cb53a313a2260f825cfef94b809ece

local-cli/templates/HelloWorld/_gitignore

@@ -40,3 +40,14 @@ buck-out/
 \.buckd/
 android/app/libs
 *.keystore
+
+# fastlane
+#
+# It is recommended to not store the screenshots in the git repo. Instead, use fastlane to re-generate the 
+# screenshots whenever they are needed.
+# For more information about the recommended setup visit:
+# https://github.com/fastlane/fastlane/blob/master/fastlane/docs/Gitignore.md
+
+fastlane/report.xml
+fastlane/Preview.html
+fastlane/screenshots