Add RTD GTFS-flex files for Call-n-Ride service and update GTFS/GTFS-…

…flex diagram (#26)

* Comparison diagram between GTFS and GTFS-flex

The diagram illustrates the major updates to the GTFS files to incorporate flex routes into a backward compatible set of GTFS-flex files.

The file is available on Google Drawings for comments/edits.

https://docs.google.com/drawings/d/1g1kuTZPLFphMa942htywksIhxXqM_mMFCROOiEw5eNo/edit?usp=sharing

* Sample of RTD's draft GTFS-flex files

The Regional Transportation District (RTD, Denver, CO, USA) has created a draft set of GTFS-flex files modeled for their Call-n-Ride service. This set of draft files is being provided here to enable users to download, review, comment, and suggest improvements both to the files, as well as the GTFS-flex specification.
More information on the Call-n-Ride service here... http://www.rtd-denver.com/callNRide.shtml