Add basic readme

Readme.md

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+# Lepidopter update repository
+
+This repository is used by the [OONI lepidopter raspberry pi
+image](https://github.com/thetorproject/lepidopter) to handle the auto update
+process.
+
+You may inspect this repository to audit what we are shipping as part of each
+update. The updates themselves are handled by means of github tags.
+
+There is a latest tag that contains a version resource as an attachment that will
+always point to the latest version.
+
+If the current version is < the latest version, we will download all the update
+scripts up until the latest and execute them in order to ensure a consistent
+update.
+