add guard example

closes #533

guards/README.md

@@ -0,0 +1,34 @@
+# guards
+
+Shows how to set up custom routing guards.
+- Routing different API versions using a header instead of path.
+
+## Usage
+
+### Running The Server
+
+```sh
+cd guards
+cargo run --bin=guards
+```
+
+### Available Routes
+
+#### `GET /api/hello`
+
+Requires the `Accept-Version` header to be present and set to `1` or `2`.
+
+Using [HTTPie]:
+
+```sh
+http :8080/api/hello Accept-Version:1
+```
+
+Using [cURL]:
+
+```sh
+curl 'localhost:8080/api/hello' -H 'accept-version: 1'
+```
+
+[HTTPie]: https://httpie.org
+[cURL]: https://curl.haxx.se