cli: Add --config option to rad self

rad.1.adoc

@@ -189,6 +189,12 @@ description and default branch by running:
 
     $ rad inspect --payload
 
+=== Configuration
+
+Radicle keeps a configuration file at *~/.radicle/config.json* by default. To
+display its exact location on your system, you can use the *rad self --config*
+command.
+
 === Environment
 
 Various environment variables are used by *rad*.