Make it easier to add custom routes

Until now, people had to edit the original route files in order to add
custom routes.

This was inconsistent with the other customizations, since we use custom
folders or files for customizing controllers, components, views, ...
(which you usually customize as well when adding a new route).

So now we're providing a file for custom routes, which will make it
easier to know which routes are not present in Consul Democracy by
default.

docs/en/customization/customization.md

@@ -12,4 +12,5 @@
 * [Other Ruby classes (GraphQL, lib, mailers, builders)](ruby.md)
 * [Gems](gems.md)
 * [Application configuration](application.md)
+* [Routes](routes.md)
 * [Tests](tests.md)