doc: write doc about creating new fetchers

README.md

@@ -28,12 +28,28 @@ Compatibility with core CKAN versions:
 
 ## Config settings
 
-
 List of config options:
 
 	# Caching strategy for chart data (required, default: redis).
 	ckanext.charts.cache_strategy = disk
 
+    # Time to live for the Redis cache in seconds. Set 0 to disable cache (default: 3600)
+    ckanext.charts.redis_cache_ttl = 7200
+
+    # Time to live for the File cache in seconds. Set 0 to disable cache.
+    ckanext.charts.file_cache_ttl = 0
+
+## Cache
+
+The extension implement a cache strategy to store the data fetched from the different sources. There are two cache strategies available: `redis` and `file`. The file cache works by storing the data in an `orc` file in the filesystem. The redis cache stores the data in a Redis database. The cache strategy can be changed at the CKAN configuration level through the admin interface or in a configuration file.
+
+## Implementing new fetchers
+
+Fetchers are responsible for fetching data from different sources (DataStore, URL, file system, hardcoded data).
+
+To register new fetchers, you need to create a new class that inherits from `DataFetcherStrategy` and implements the `fetch_data` and `make_cache_key` methods.
+The `fetch_data` method should return a `pandas` `DataFrame` object with the data that should be displayed in the chart.
+The `make_cache_key` method should return a unique string that will be used as a key to store the data in the cache.
 
 ## Developer installation
 

ckanext/charts/config_schema.yaml

@@ -16,10 +16,10 @@ fields:
 
   - field_name: ckanext.charts.redis_cache_ttl
     label: Redis Cache TTL
-    help_text: Time to live for the cache in seconds. Set 0 to disable cache.
+    help_text: Time to live for the Redis cache in seconds. Set 0 to disable cache.
     input_type: number
 
   - field_name: ckanext.charts.file_cache_ttl
     label: File Cache TTL
-    help_text: Time to live for the cache in seconds. Set 0 to disable cache.
+    help_text: Time to live for the File cache in seconds. Set 0 to disable cache.
     input_type: number