Add rate limits documentation

doc/rate_limits.md

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+# Rate limits
+
+Services can be rate-limited. (currently only gecoding with Mapzen or Here providers is limited)
+
+The rate limits configuration can be established at server, organization or user levels, the latter having precedence over the earlier.
+
+The default configuration (a null or empty configuration) doesn't impose any limits.
+
+The configuration consist of a JSON object with two attributes:
+
+* `period`: the rate-limiting period, in seconds.
+* `limit`: the maximum number of request in the established period.
+
+If a service request exceeds the configured rate limits
+(i.e. if more than `limit` calls are performe in a fixed interval of
+duration `period` seconds) the call will fail with an "Rate limit exceeded" error.
+
+## Server-side interface
+
+There's a server-side SQL interface to query or change the configuration.
+
+### cdb_dataservices_server.cdb_service_get_rate_limit(username, orgname, service)
+
+This function returns the rate limit configuration for a given user and service.
+
+#### Returns
+
+The result is a JSON object with the configuration (`period` and `limit` attributes as explained above).
+
+### cdb_dataservices_server.cdb_service_set_user_rate_limit(username, orgname, service, rate_limit)
+
+This function sets the rate limit configuration for the user. This overrides any other configuration.
+
+The configuration is provided as a JSON literal. To remove the user-level configuration `NULL` should be passed as the `rate_limit`.
+
+#### Returns
+
+This functions doesn't return any value.
+
+### cdb_dataservices_server.cdb_service_set_org_rate_limit(username, orgname, service, rate_limit)
+
+This function sets the rate limit configuration for the organization.
+This overrides server level configuration and is overriden by user configuration if present.
+
+The configuration is provided as a JSON literal. To remove the organization-level configuration `NULL` should be passed as the `rate_limit`.
+
+#### Returns
+
+This functions doesn't return any value.
+
+### cdb_dataservices_server.cdb_service_set_server_rate_limit(username, orgname, service, rate_limit)
+
+This function sets the default rate limit configuration for all users accesing the dataservices server. This is overriden by organization of user configuration.
+
+The configuration is provided as a JSON literal. To remove the organization-level configuration `NULL` should be passed as the `rate_limit`.
+
+#### Returns
+
+This functions doesn't return any value.
+
+## Client-side interface
+
+For convenience there's also a client-side interface (in the client dataservices-api extension), consisting
+of public functions to get the current configuration and privileged functions to change it.
+
+### Public functions
+
+### cdb_dataservices_client.cdb_service_get_rate_limit(service)
+
+This function returns the rate limit configuration for the specified service
+and the user corresponding to the role which makes the calls.
+
+#### Returns
+
+The result is a JSON object with the configuration (`period` and `limit` attributes as explained above).
+
+### Privileged function
+
+Thes functions are not accessible by regular user roles, and the user and organization names must be provided as parameters.
+
+### cdb_dataservices_server.cdb_service_set_user_rate_limit(username, orgname, service, rate_limit)
+
+This function sets the rate limit configuration for the user. This overrides any other configuration.
+
+The configuration is provided as a JSON literal. To remove the user-level configuration `NULL` should be passed as the `rate_limit`.
+
+#### Returns
+
+This functions doesn't return any value.
+
+### cdb_dataservices_server.cdb_service_set_org_rate_limit(username, orgname, service, rate_limit)
+
+This function sets the rate limit configuration for the organization.
+This overrides server level configuration and is overriden by user configuration if present.
+
+The configuration is provided as a JSON literal. To remove the organization-level configuration `NULL` should be passed as the `rate_limit`.
+
+#### Returns
+
+This functions doesn't return any value.
+
+### cdb_dataservices_server.cdb_service_set_server_rate_limit(username, orgname, service, rate_limit)
+
+This function sets the default rate limit configuration for all users accesing the dataservices server. This is overriden by organization of user configuration.
+
+The configuration is provided as a JSON literal. To remove the organization-level configuration `NULL` should be passed as the `rate_limit`.
+
+#### Returns
+
+This functions doesn't return any value.