Crashlytics add warning about including new and old SDK (#4755)

Author: samedson

Crashlytics/Crashlytics/Public/FIRCrashlytics.h

@@ -14,6 +14,12 @@
 
 #import <Foundation/Foundation.h>
 
+#if __has_include(<Crashlytics/Crashlytics.h>)
+#warning "FirebaseCrashlytics and Crashlytics are not compatible \
+in the same app because including multiple crash reporters can \
+cause problems when registering exception handlers."
+#endif
+
 NS_ASSUME_NONNULL_BEGIN
 
 /**