docs: small usage section mentioning CLI

docs/index.rst

@@ -82,6 +82,7 @@ Table of Contents
    tutorial
    installation
    configuration
+   usage
    implementation
    changelog
    GitHub Repository & Issue Tracker <https://github.com/zrepl/zrepl>

docs/usage.rst

@@ -0,0 +1,28 @@
+*****
+Usage
+*****
+
+============
+CLI Overview
+============
+
+.. NOTE::
+
+    To avoid duplication, the zrepl binary is self-documenting:
+    invoke any subcommand at any level with the ``--help`` flag to get information on the subcommand, available flags, etc.
+
+.. list-table::
+    :widths: 30 70
+    :header-rows: 1
+
+    * - Subcommand
+      - Description
+    * - ``zrepl daemon``
+      - run the daemon, required for all zrepl functionality
+    * - ``zrepl control``
+      - control / query the daemon
+    * - ``zrepl control status``
+      - show job activity / monitoring (``--format raw``)
+    * - ``zrepl stdinserver``
+      - see :ref:`transport-ssh+stdinserver`
+