Added clean

git-svn-id: https://nagiosplug.svn.sourceforge.net/svnroot/nagiosplug/nagiosplug/trunk@935 f882894a-f735-0410-b71e-b25c423dba1c

doc/makefile

@@ -6,3 +6,6 @@ developer-guidelines.html: developer-guidelines.sgml
 		mv developer-guidelines/developer-guidelines.html . ;\
 		rm -f developer-guidelines ;\
 	fi
+
+clean:
+	rm -f developer-guidelines.html