Dangerfile: warn about too many commits.

Addressed to #537

Author: php-coder

Diff for: Dangerfile

@@ -668,6 +668,16 @@ else
 	end
 end
 
+commits = git.commits.size
+if commits > 1
+	warn(
+		"danger check: pull request contains #{commits} commits while most of the cases it should have only one.\n"\
+		"If it's not a special case you should squash commits into single one.\n"\
+		"You can read how to do it here: https://davidwalsh.name/squash-commits-git\n"\
+		"But be careful because **it can destroy** all your changes!"
+	)
+end
+
 all_checks_passed = violation_report[:errors].empty? && violation_report[:warnings].empty? && violation_report[:messages].empty?
 if all_checks_passed
 	message(