Added README

Signed-off-by: Louis Vallat <louis@louis-vallat.xyz>

README.md

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+# Gitlab CI Launchpad Mini Controller
+
+> And it's in Rust 🦀!
+
+![Launchpad Mini](https://imgur.com/sdLy3XK.png)
+
+## Aim
+
+I wanted to use my Launchpad Mini in a way that was original and useful to me,
+as I don't produce music. The Launchpad is a glorified MIDI pad with some LEDs,
+and as a MIDI device, it can be interfaced with to do pretty much anything with
+it.
+
+I used mainly two libraries for this project:
+
+- [gitlab](https://crates.io/crates/gitlab) to interact with the Gitlab API
+- [launchy](https://crates.io/crates/launchy) to interact with the Launchpad
+
+These two libraries are maintained at the moment this README is being written.
+
+## Building
+
+As any other `cargo` project, it can be built with a simple command:
+
+```bash
+cargo build --release
+```
+
+## Configuring
+
+The configuration is done through a JSON file, named `config.json` and located
+in the current working directory. An example configuration file is provided,
+`config.json.example`.
+
+The abs\_x and abs\_y coordinate are defined using the top-left grid tile as
+the origin.
+
+Clicking on a tile opens the corresponding web page for the project linked to
+this tile. Clicking on the `A` button will light it up, it is the restart button.
+If you click on a tile with the `A` button lit up, it will retry or create a
+new pipeline for this project and for this ref. You can disengage the restart
+mode by pressing again the `A` button.
+
+## Limitations
+
+This project has some limitations right now, and some of them will be fixed:
+
+- the program is able to talk to one and only one gitlab API right now
+- the configuration file has to be in the current working directory
+- only one page is allowed, but 8 could be leveraged later using the 8 selectors
+