Add example for fetching mutating datasets

doc/usage.rst

@@ -138,6 +138,52 @@ Alternative hashing algorithms supported by :mod:`hashlib` can be used if necess
     print(pooch.file_hash("data/c137.csv", alg="sha512"))
 
 
+Bypassing the hash check
+------------------------
+
+Sometimes we might not know the hash of the file or it could change on the server
+periodically. In these cases, we need a way of bypassing the hash check.
+One way of doing that is with Python's ``unittest.mock`` module. It defines the object
+``unittest.mock.ANY`` which passes all equality tests made against it. To bypass the 
+check, we can set the hash value to ``unittest.mock.ANY`` when specifying the 
+``registry`` argument for :func:`pooch.create`.
+
+In this example, we want to use Pooch to download a list of weather stations around 
+Australia. The file with the stations is in an FTP server and we want to store it locally 
+in separate folders for each day that the code is run. The problem is that the 
+``stations.zip`` file is updated on the server instead of creating a new one, so the 
+hash check would fail. This is how you can solve this problem:
+
+
+.. code:: python
+
+    import datetime
+    import unittest.mock  
+    import pooch
+
+    # Get the current data to store the files in separate folders
+    CURRENT_DATE = datetime.datetime.now().date()
+
+    GOODBOY = pooch.create(
+        path=pooch.os_cache("bom_daily_stations") / CURRENT_DATE,
+        base_url="ftp://ftp.bom.gov.au/anon2/home/ncc/metadata/sitelists/",
+        # Use ANY for the hash value to ignore the checks
+        registry={
+            "stations.zip": unittest.mock.ANY,
+        },
+    )
+
+Because hash check is always ``True``, Pooch will only download the file once. When running 
+again at a different date, the file will be downloaded again because the local cache folder 
+changed and the file is no longer present in it. If you omit ``CURRENT_DATE`` from the cache 
+path, then Pooch will only fetch the files once, unless they are deleted from the cache.
+
+.. note:: 
+
+    If run over a period of time, your cache directory will increase in size, as the 
+    files are stored in daily subdirectories.
+
+
 Versioning
 ----------