add Denver RTD resource

README.md

@@ -461,6 +461,7 @@ Please note a passing build status indicates all listed APIs are available since
 | Transport for Budapest, Hungary | Budapest public transport API | No | Yes | [Go!](https://apiary.io/) |
 | Transport for Chicago, US | CTA | No | No | [Go!](http://www.transitchicago.com/developers/) |
 | Transport for Czech Republic | Czech transport API | No | Yes | [Go!](https://www.chaps.cz/eng/products/idos-internet) |
+| Transport for Denver, US | RTD | No | No | [Go!](http://www.rtd-denver.com/gtfs-developer-guide.shtml#realtime-feeds) |
 | Transport for Finland | Finnish transport API | No | Yes | [Go!](https://digitransit.fi/en/developers/ ) |
 | Transport for Germany | Deutsche Bahn (DB) API | `apiKey` | No | [Go!](http://data.deutschebahn.com/dataset/api-fahrplan) |
 | Transport for India | India Public Transport API | `apiKey` | Yes | [Go!](https://data.gov.in/sector/transport) |