Proposal for new field: route.route_sort_order (#83)

* Add field route_sort_order

* Specify ordering for route_sort_order

gtfs/spec/en/reference.md

@@ -147,6 +147,7 @@ File: **Required**
 |  route_url | Optional | The **route_url** field contains the URL of a web page about that particular route. This should be different from the agency_url.  The value must be a fully qualified URL that includes **http**:// or **https**://, and any special characters in the URL must be correctly escaped. See http://www.w3.org/Addressing/URL/4_URI_Recommentations.html for a description of how to create fully qualified URL values. |
 |  route_color | Optional | In systems that have colors assigned to routes, the **route_color** field defines a color that corresponds to a route. The color must be provided as a six-character hexadecimal number, for example, 00FFFF. If no color is specified, the default route color is white (FFFFFF).  The color difference between **route_color** and **route_text_color** should provide sufficient contrast when viewed on a black and white screen. The [W3C Techniques for Accessibility Evaluation And Repair Tools document](https://www.w3.org/TR/AERT#color-contrast) offers a useful algorithm for evaluating color contrast. There are also helpful online tools for choosing contrasting colors, including the [snook.ca Color Contrast Check application](http://snook.ca/technical/colour_contrast/colour.html#fg=33FF33,bg=333333). |
 |  route_text_color | Optional | The route_text_color field can be used to specify a legible color to use for text drawn against a background of route_color. The color must be provided as a six-character hexadecimal number, for example, FFD700. If no color is specified, the default text color is black (000000).  The color difference between **route_color** and **route_text_color** should provide sufficient contrast when viewed on a black and white screen. |
+|  route_sort_order | Optional | The **route_sort_order** field can be used to order the routes in a way which is ideal for presentation to customers. It must be a non-negative integer. Routes with smaller **route_sort_order** values should be displayed before routes with larger **route_sort_order** values. |
 
 ### trips.txt