[docs] Note StrictSlash re-direct behaviour #308 (#321)

* [docs] Note StrictSlash re-direct behaviour #308

* StrictSlash enabled routes return a 301 to the client
* As per the HTTP standards, non-idempotent methods, such as POST or PUT, will be followed with a GET by the client
* Users should use middleware if they wish to change this behaviour to return a HTTP 308.

* Update description of StrictSlash

mux.go

@@ -164,13 +164,18 @@ func (r *Router) GetRoute(name string) *Route {
 // StrictSlash defines the trailing slash behavior for new routes. The initial
 // value is false.
 //
-// When true, if the route path is "/path/", accessing "/path" will redirect
+// When true, if the route path is "/path/", accessing "/path" will perform a redirect
 // to the former and vice versa. In other words, your application will always
 // see the path as specified in the route.
 //
 // When false, if the route path is "/path", accessing "/path/" will not match
 // this route and vice versa.
 //
+// The re-direct is a HTTP 301 (Moved Permanently). Note that when this is set for
+// routes with a non-idempotent method (e.g. POST, PUT), the subsequent re-directed
+// request will be made as a GET by most clients. Use middleware or client settings
+// to modify this behaviour as needed.
+//
 // Special case: when a route sets a path prefix using the PathPrefix() method,
 // strict slash is ignored for that route because the redirect behavior can't
 // be determined from a prefix alone. However, any subrouters created from that