Add coding conventions in contribute.md

doc/contribute.md

@@ -21,4 +21,24 @@ 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.
+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>`