Add Contribution guide

CONTRIBUTING.md

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+# Contributing Guide
+
+## Issue contributions
+For either questions or bugs, submit a ticket for your issue, assuming one does
+not already exist. Try to:
+
+* Describe the issue including steps to reproduce when it is a bug.
+* Make sure you fill in the version that you are using.
+
+## Code contributions
+If you want to contribute, fork this project and make a pull request. Try to:
+
+* Write tests.
+* Follow our [coding guidelines][coding].
+* Write a [good commit message][commit].
+
+[coding]: doc/guides/coding-guidelines.md
+[commit]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html

doc/guides/coding-guidelines.md

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+# Coding guidelines
+
+## Names
+1. Use *DLB_PascalCase* for C interface function names.
+1. Use *snake_case* for function names.
+1. Use *snake_case* for local variables.
+1. Use *UPPER_CASE* for enum values.
+1. Use whole words in names when possible.
+
+## Style
+1. Use 4 spaces per indentation.
+1. Braces should start on the same line as the statement.
+1. Always surround loop and conditional bodies with curly braces.
+    Statements on the same line are allowed to omit braces.
+1. Type names should (in general) be defined inside the scope that uses them.
+1. Error condition should be reported through return enum mechanism.
+
+## Functions
+1. Use static functions when possible.
+1. Do not export a symbol name if it does not belong to the interface.
+
+## Documentation
+1. Use doxygen to document all functions that belong to the interface.
+    * A brief summary (one sentence) declaring the function main purpose
+        (mandatory).
+    * A list of parameters (with its type [in], [out] or [in,out]) including a
+        short description (mandatory, if any).
+    * The returning value of the function (mandatory, if non-void).
+    * A more detailed function description (when needed).
+    * A doxygen example:
+
+        ```c
+        /*! \brief Brief function description
+         *! \param[type] param1 Param 1 description
+         *! \param[type] param2 Param 2 description
+         *! \return Return value description
+         */
+        int DLB_Function(type1_t param1, type2_t, param2) {
+            ...
+        }
+        ```
+
+## License
+1. All files (sources, headers, scripts, ...) MUST have the proper license
+header
+