Add link header to APIv3 for the spec file

lib/api/v3/root.rb

@@ -37,8 +37,11 @@ module API
     class Root < ::API::OpenProjectAPI
       helpers ::API::V3::Utilities::EpropsConversion
 
-      # All endpoint accept query props as gzipped and base64 encoded json objects
       before do
+        # Add Link header for openapi spec
+        header 'Link', '</api/v3/openapi.json>; rel="service-desc"'
+
+        # All endpoint accept query props as gzipped and base64 encoded json objects
         transform_eprops
       end
 
@@ -86,6 +89,10 @@ module API
         API::OpenAPI.spec
       end
 
+      get '/openapi.json' do
+        API::OpenAPI.spec
+      end
+
       get '/spec.yml' do
         content_type 'text/vnd.yaml'