Write the coding-style guide for Modmesh

contrib/CODINGSTYLE.md

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+# Coding Style
+## Introduction
+This is the coding style guide for Modmesh.  
+* [Introduction](#introduction)
+* [Naming](#naming)
+* [Comments](#comments)
+* [Formatting](#formatting)
+* [Class](#class)
+* [Functions](#functions)
+
+### Naming
+1. The class names use Camel case, such as ```class R3DWidget;```. 
+2. The name of the variable / using-declarations uses snake case,  
+such as ```R3DWidget * viewer;``` and ```using size_type = std::size_t;```. 
+3. The QT-related function name uses Camel case,  
+whereas non-QT-related function name uses snake case.
+4. The name of the class member starts with ```m_```.
+5. The name of [macros](https://en.cppreference.com/w/cpp/preprocessor/replace) starts with ```DECL_MM_```.
+6. The [using-declarations](https://en.cppreference.com/w/cpp/language/using_declaration) ends with ```_type```.
+
+### Comments
+1. The comment blocks follow the [doxygen styles](https://www.doxygen.nl/manual/docblocks.html).  
+2. The one-line comment, ```/* end of NamespaceOrClassName */```,  
+should be appended to the end of a namespace or class.  
+For example :
+```c++
+namespace modmesh
+{
+// More code...
+}; /* end of modmesh */
+```
+3. It is highly recommended to add the reference in the comment block.
+
+### Formatting
+1. It is necessary to obey the [clang-format](https://clang.llvm.org/docs/index.html), or you must fail the github action.  
+2. There exists a space between the definition of the classes/functions.
+3. A long line should be devided to several shorter lines.
+
+### Class
+1. The class should follow [the rule of five](https://en.cppreference.com/w/cpp/language/rule_of_three).
+
+### Functions
+1. The recursive function should be replaced with the iterative function, unless it is inevitable to be used.