update getting started

Author: olegantonyan

docs/getting_started.md

@@ -1 +1,14 @@
-# WIP
+# Installing omnipackage-agent
+
+Agent is responsible for building packages. You'll need it to build packages locally.
+
+## Multiple ways to install it
+
+1. OmniPackage repositories [web.omnipackage.org/oleg/omnipackage-agent](https://web.omnipackage.org/oleg/omnipackage-agent).
+2. [Source code](https://github.com/omnipackage/omnipackage-agent-ruby). Reasonable recent version of Ruby is required, as well as docker or podman. Executables located in `exe/` directory.
+
+## Test your installation
+
+```shell
+omnipackage --version
+```