Add documentation about the quota information functions

See #331

doc/quota_information.md

@@ -12,3 +12,133 @@ Quota consumption is calculated based on the number of request made for each fun
   **Note:** Queries to the Data Services API, and any of its functions in your maps, may be forbidden in the future
 
 * It is advised to store results of these queries into your datasets, and refresh them as needed. This ensure more control of quota credits for your account
+
+
+## Quota information functions
+
+Two functions are available to obtain information about available services quotas. 
+
+## cdb_service_quota_info()
+
+Returns information about per-service quotas (available and used) for the account.
+
+#### Returns
+
+This function returns a set of service quota information records, one per service.
+
+Name            | Type      | Description
+--------------- | --------- | ------------
+`service`       | `text`    | Type of service.
+`monthly_quota` | `numeric` | Quota available to the user (number of calls) per monthly period.
+`used_quota`    | `numeric` | Quota used by the user in the present period.
+`soft_limit`    | `boolean` | True if the user has *soft-limit* quota
+`provider`      | `text`    | Service provider for this type of service
+
+Service types:
+
+* `'isolines'` [Isoline/Isochrones (isochrone/isodistance lines) service](https://carto.com/docs/carto-engine/dataservices-api/isoline_functions/)
+* `'hires_geocoder'` [Street level geocoding](https://carto.com/docs/carto-engine/dataservices-api/geocoding-functions#street-level-geocoder)
+* `'routing'` [Routing functions](https://carto.com/docs/carto-engine/dataservices-api/routing_functions/)
+* `'observatory'` Data Observatory services ([demographic](https://carto.com/docs/carto-engine/dataservices-api/demographic_functions/) and [segmentation](https://carto.com/docs/carto-engine/dataservices-api/segmentation_functions/) functions)
+
+Notes:
+
+Users who have *soft-quota* activated never run out of quota, but they may incurr into extra
+expenses when the regular quota is exceeded.
+
+A zero value of `monthly_quota` indicates that the service has not been activated for the user.
+
+#### Example
+
+```sql
+SELECT * FROM cdb_service_quota_info();
+```
+
+Result:
+
+```
+    service     | monthly_quota | used_quota | soft_limit |     provider
+----------------+---------------+------------+------------+------------------
+ isolines       |           100 |          0 | f          | mapzen
+ hires_geocoder |           100 |          0 | f          | mapzen
+ routing        |            50 |          0 | f          | mapzen
+ observatory    |             0 |          0 | f          | data observatory
+(4 rows)
+
+```
+
+In this case we notice the user has no access to the observatory services, 
+all quotas are *hard-limited* (no soft limits), and no quota has been used 
+in the present period.
+
+## cdb_enough_quota(service text ,input_size numeric)
+
+This function is useful to check if enough quota is available for completing a job.
+
+This is specially relevant if a number of service calls are to be performed inside a transaction:
+if any of the calls fails due to exceeded quota the transaction will be rolled back, resulting
+in some quota being consumed but no saved results from the services consumed.
+
+In the case of of calling repeatedly quota-consuming functions (e.g. to geocode a whole table) it is 
+extremely important to first check if enough quota is available to complete the job using
+this function.
+
+#### Arguments
+
+Name         | Type      | Description
+------------ | --------- | -----------
+`service`    | `text`    | Service to check; see the list of valid services above
+`input_size` | `numeric` | Number of service calls required, i.e. size of the input to be processed
+
+#### Returns
+
+The result is a *boolean* value. A *true* value (`'t'`) indicates that the available quota
+for the service is enough for the input size requested. A *false* value (`'f'`) indicates
+insufficient quota.
+
+#### Example
+
+Imagine you wish to geocode a table named 
+To check if you have enogh quota is available and avoiding ending with a partially
+
+First you should find out how many records you need to geocode:
+
+```sql
+SELECT COUNT(*) FROM {tablename} WHERE {street_name_column} IS NOT NULL;
+```
+
+Result: here's a sample result of 10000 records:
+
+```
+ count
+-------
+  10000
+(1 row)
+```
+
+Now you can find out if there's enough quota to complete this job
+
+```sql
+SELECT cdb_enough_quota('hires_geocoder', {number_of_records});
+```
+
+The result should be similar to this:
+
+```
+ cdb_enough_quota
+------------------
+ t
+```
+
+If the result of this query is *true* (`'t'`) you can safely proceed; if you get
+a *false* value (`'f'`) you should avoid the processing; use `cdb_service_quota_info` as explained
+above to obtain further information.
+
+Don't forget to apply any filtering conditions you've used to 
+count the records (in our case `{street_name_column} IS NOT NULL`):
+
+
+```sql
+UPDATE {tablename} SET the_geom = cdb_geocode_street_point({street_name_column})
+  WHERE {street_name_column} IS NOT NULL;
+```