Add Usage section to readme.

README.adoc

@@ -19,6 +19,29 @@
 *{project}* turns form data into JSON and opens a pull request on
 link:https://schlomp.space/FediBlock/data[FediBlock/data].
 
+== Usage
+
+.nginx config
+[source,nginx]
+--------------------------------------------------------------------------------
+# […]
+
+location /add {
+    include /etc/nginx/fastcgi_params;
+    fastcgi_pass    unix:/var/run/cgi-fcgiwrap.socket-1;
+    fastcgi_param SCRIPT_FILENAME /usr/bin/fediblock-backend;
+    fastcgi_param HOME "/var/lib/nginx"; # When I didn't set it, HOME was /root.
+}
+--------------------------------------------------------------------------------
+
+.Generate SSH key
+[source,shell]
+--------------------------------------------------------------------------------
+sudo -u nginx ssh-keygen -t ed25519 -f ~nginx/.local/share/fediblock-backend/ssh_id
+--------------------------------------------------------------------------------
+
+Don't forget to add the key to the data repo.
+
 == Install
 
 === From source