Updated GBFS language at root, provider, & agency

Author: schnuerle

agency/README.md

@@ -12,6 +12,7 @@ This specification contains a collection of RESTful APIs used to specify the dig
   * [Versioning](#versioning)
   * [Responses and Error Messages](#responses-and-error-messages)
   * [Authorization](#authorization)
+  * [GBFS](#gbfs)
 * [Vehicles](#vehicles)
   * [Vehicle - Register](#vehicle---register)
   * [Vehicle - Update](#vehicle---update)
@@ -36,6 +37,12 @@ Versioning must be implemented as specified in the [Versioning section][versioni
 
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+### Modes
+
+MDS is intended to be used for multiple transportation modes, including its original micromobility (e-scooters, bikes, etc.) as well as additional modes such as taxis and delivery bots.  A given `provider_id` shall be associated with a single mobility [mode], so that the mode does not have to be specified in each data structure and API call.  A provider implementing more than one mode shall [register](/README.md#providers-using-mds) a `provider_id` for each mode.
+
+[Top][toc]
+
 ### Responses and Error Messages
 
 See the [Responses][responses] and [Error Messages][error-messages] sections.
@@ -48,6 +55,12 @@ When making requests, the Agency API expects `provider_id` to be part of the cla
 
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+### GBFS
+
+See the [GBFS Requirement](/README.md#gbfs-requirement) language for more details.
+
+[Top][toc]
+
 ## Vehicles
 
 The `/vehicles` endpoint returns the specified vehicle (if a device_id is provided) or a list of known vehicles. Providers can only retrieve data for vehicles in their registered fleet.