doc: update and improve readme

README.md

@@ -8,7 +8,7 @@ Code de la tourelle de pointage laser du projet bouchons EDF.
 > [!IMPORTANT]
 > Nouvelle méca, les axes H/x et V/y sont découplés.
 
-# Usage
+## Usage
 
 Compile through Arduino IDE or equivalent.
 
@@ -16,6 +16,22 @@ Compile through Arduino IDE or equivalent.
 > Add [kissStepper](https://github.com/risitt/kissStepper) to your project
 > librairies.
 
+## Conventions
+
+- **x** is the horizontal axis centered at 0 and going from left (min value) to
+  right (max value).
+- **y** is the vertical axis centered at 0 and going from bottom (min value) to
+  top (max value).
+- **z** is the distance between the turrent and the target.
+
+```mermaid
+xychart-beta
+  title "Turret coordinate system"
+  x-axis "X axis [m]" -3 --> 3 
+  y-axis "Y axis [m]" -3 --> 3
+  bar [-4]
+```
+
 ## Development
 
 Required dependeinces if not using