Add coding conventions in contribute.md

doc/contribute.md

@@ -22,3 +22,23 @@ Then, you can submit a pull-request for review. Try to describe your pull reques
 Other contributors can post comments about the pull request, maybe ask for more info or adjustements in the code.
 
 Once the pull request is reviewed an accepted, it'll be merge in **develop** and will be released in the next release version of the firmware.
+
+# Coding convention
+## Language
+The language of this project is **C++**, and all new code must be written in C++. (Modern) C++ provides a lot of useful tools and functionalities that are beneficial for embedded software development like `constexpr`, `template` and anything that provides zero-cost abstraction.  
+
+It's OK to include C code if this code comes from another library like FreeRTOS, NimBLE, LVGL or the NRF-SDK. 
+
+## Coding style
+The most important rule to follow is to try to keep the code as easy to read and maintain as possible. 
+
+ - **Identation** : 2 spaces, no tabulation
+ - **Opening brace** at the end of the line
+ - **Naming** : Choose self-describing variable name
+    - **class** : PascalCase
+    - **namespace** : PascalCase
+    - **variable** : camelCase, **no** prefix/suffix ('_', 'm_',...) for class members
+ - **Include guard** : `#pragma once` (no `#ifdef __MODULE__ / #define __MODULE__ / #endif`)
+ - **Includes** :
+    - files from the project : `#include "relative/path/to/the/file.h"`
+    - external files and std : `#include <file.h>`