Document post_install workaround for static library source pod usage (#534)

Author: paulb777

README.md

@@ -51,7 +51,10 @@ If your Podfile does not include *use_frameworks!*, you need to workaround
 a build issue with the FirebaseAnalytics umbrella header. Delete the first four lines
 of Pods/FirebaseAnalytics/Frameworks/FirebaseAnalytics.framework/Headers/FirebaseAnalytics.h
 or copy [patch/FirebaseAnalytics.h](patch/FirebaseAnalytics.h) to
-Pods/FirebaseAnalytics/Frameworks/FirebaseAnalytics.framework/Headers/FirebaseAnalytics.h
+Pods/FirebaseAnalytics/Frameworks/FirebaseAnalytics.framework/Headers/FirebaseAnalytics.h.
+See the post_install phase of [Example/Podfile](Example/Podfile) for an example
+of applying the workaround automatically - make sure you correct the path of
+`patch/FirebaseAnalytics.h`.
 
 ## Usage