Add firewall instructions to README

Author: goerz

README.md

@@ -47,10 +47,12 @@ served (e.g. a private SSH key)
 *  Python >= 3.5
 *  [click package][2]
 *  A server that is accessible through a public hostname, with GPG installed
-   and the private key for the KEYID that is to be exported in its keychain
+   and the private key for the KEYID that is to be exported in its keychain.
+   Check available private keys with `gpg2 -K`.
 *  SSL certificates for the public hostname. It is recommended to use
    [Let's Encrypt][3]. You may use an existing certificate for a webserver
-   running on the host
+   running on the host. Since `gpg-serve-key` will run on a non-standard port,
+   it will not be necessary to temporarily suspend the web server.
 
 
 ## Usage ##
@@ -62,15 +64,26 @@ Run the script directly as e.g.
         --key-file=/etc/letsencrypt/live/michaelgoerz.net/privkey.pem \
         --host=michaelgoerz.net 57A6CAA6
 
-See `./gpg-serve-key --help` for more details.
+See `./gpg-serve-key --help` for more details. You may use either the short
+8-digit key KEYID, or the full length KEYID as shown by `gpg -K`.
 
-This will start temporary webserver at a random port and serve both the public
-and the private key at URLs such as
+The command will start a temporary webserver at a random port and serve both
+the public and the private key at URLs such as
 
     https://michaelgoerz.net:47409/v1f4Y7XixMQ/57A6CAA6-public.key
     https://michaelgoerz.net:47409/v1f4Y7XixMQ/57A6CAA6-secret.key
 
-After importing the keys from these URLs, stop the server by hitting `ctrl+c`.
+If using a Cloudflare proxy for the domain, it must be temporarily disabled.
+Make sure any firewall running on the server is set up allow access to the
+port. On Ubuntu, to allow access to, e.g., port `47409`, run
+
+    sudo ufw allow 47409
+
+After importing the keys from the above URLs, stop the server by hitting
+`ctrl+c`.
+
+If applicable, remove the firewall rule (`sudo ufw delete allow 47409`), and
+re-enable the Cloudflare proxy.
 
 [1]: https://mssun.github.io/passforios/
 [2]: http://click.pocoo.org/5/