OSS exclude Xcode artifacts in ReactCommon subdir

Summary: ReactCommon subdir is supposed to be focused on C++, so Xcode projects shouldn't creep in.

Reviewed By: PeteTheHeat

Differential Revision: D7939858

fbshipit-source-id: 2e90491bdaec92829485aded071f74d004b24b21

Author: fkgozali

.gitignore

@@ -66,3 +66,7 @@ package-lock.json
 
 /coverage
 /third-party
+
+# ReactCommon subdir shouldn't have Xcode project
+/ReactCommon/**/*.xcodeproj
+