document non-capturing groups (closes #143)

doc.go

@@ -47,6 +47,10 @@ variable will be anything until the next slash. For example:
 	r.HandleFunc("/articles/{category}/", ArticlesCategoryHandler)
 	r.HandleFunc("/articles/{category}/{id:[0-9]+}", ArticleHandler)
 
+Groups can be used inside patterns, as long as they are non-capturing (?:re). For example:
+
+	r.HandleFunc("/articles/{category}/{sort:(?:asc|desc|new)}", ArticlesCategoryHandler)
+
 The names are used to create a map of route variables which can be retrieved
 calling mux.Vars():