Update readme. (#2)

README.md

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 # rosdistro-snapshots
-Snapshot tags for ros/rosdistro
+
+This repo is part of a larger demonstration of Clearpath's effort to build
+and deploy ROS packages using Nix, and has been prepared and released
+publicly as part of a ROSCon talk given in Kyoto, in October 2022.
+
+The repo houses a GitHub Action which creates daily snapshot tags of the devel
+sources in https://github.com/ros/rosdistro, as well as those snapshots
+themselves.
+
+The latest one can be found in the orphan `snapshot/latest` branch.
+
+To follow the self-guided demonstration and find links to other related
+resources, please see the main repository:
+
+https://github.com/clearpathrobotics/nix-base