new: doc: [Hedgedoc] add README to manage users with cli tool

hedgedoc/README.org

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+# -*- ispell-local-dictionary: "english" -*-
+
+* Usage
+
+** How to manage users
+
+This allows to reset password for users.
+
+From container (using `docker exec -ti MY_CONTAINER sh`), use
+`/hedgedoc/bin/manage_users` command (it is not in $PATH).
+
+#+begin_example
+Command-line utility to create users for email-signin.
+
+Usage: bin/manage_users [--pass password] (--add | --del) user-email
+        Options:
+                --add   Add user with the specified user-email
+                --del   Delete user with specified user-email
+                --reset Reset user password with specified user-email
+                --pass  Use password from cmdline rather than prompting
+#+end_example
+