Merge pull request NixOS#5 from nh2/faq-cache-refresh

FAQ: Re-checking binary caches

source/faq.rst

@@ -216,7 +216,18 @@ Using `Nix`:
     $ nix-build helpers/bench.nix --option extra-binary-caches https://hydra.snabb.co`
 
 
+How do I force nix to re-check whether something exists at a binary cache?
+--------------------------------------------------------------------------
 
+Nix caches the contents of binary caches so that it doesn't have to query them
+on every command. This includes negative answers (cache doesn't have something).
+The default timeout for that is 1 hour as of writing.
 
+To wipe all cache-lookup-caches:
 
+.. code-block:: bash
+
+    $ rm $HOME/.cache/nix/binary-cache-v5.sqlite*
 
+Alternatively, use the ``narinfo-cache-negative-ttl`` option to reduce the
+cache timeout.