ngclient: Add note about RequestsFetcher being deprecated

Signed-off-by: Jussi Kukkonen <[email protected]>

Author: jku

tuf/ngclient/requests_fetcher.py

@@ -3,6 +3,13 @@
 
 """Provides an implementation of ``FetcherInterface`` using the Requests HTTP
 library.
+
+Note that this module is deprecated, and the default fetcher is
+Urllib3Fetcher:
+* RequestsFetcher is still available to make it easy to fall back to
+    previous implementation if issues are found with the Urllib3Fetcher
+* If RequestsFetcher is used, note that `requests` must be explicitly
+    dependended on: python-tuf does not do that.
 """
 
 from __future__ import annotations