Add a usecase description to README

Signed-off-by: Igor Shishkin <me@teran.dev>

README.md

@@ -6,3 +6,13 @@
 
 Trivial library, example server and client applications to provide secrets for
 applications from third party password manager authenticated once.
+
+## Use case
+
+Secretbox provides ability to run any other application which can obtain
+secrets from a command (like [restic](https://restic.net) or
+[rclone](https://rclone.org)) to get the secrets it needs with one-time
+token through unix-socket. In that scenario service side must be used
+to serve unix-socket connections by application runner.
+
+Examples can be found in cmd: cli and server side.