Add example MatcherFunc

doc.go

@@ -87,7 +87,9 @@ There are several other matchers that can be added. To match path prefixes:
 
 ...or to use a custom matcher function:
 
-	r.MatcherFunc(myFunc)
+	r.MatcherFunc(func(r *http.Request, rm *RouteMatch) bool {
+		return r.ProtoMajor == 0
+        })
 
 ...and finally, it is possible to combine several matchers in a single route: