Documentation to improve pod gen usability (#4041)

Author: paulb777

README.md

@@ -81,6 +81,9 @@ For the pod that you want to develop:
 
 `pod gen Firebase{name here}.podspec --local-sources=./ --auto-open`
 
+Note: If the CocoaPods cache is out of date, you may need to run
+`pod repo update` before the `pod gen` command.
+
 Firestore has a self contained Xcode project. See
 [Firestore/README.md](Firestore/README.md).