Add contribution guide

CONTRIBUTING.md

+# Contributing to `pugs`
+
+----
+## Branches
+
+`develop` and `master` branches are protected. This means that one cannot `push` on them, so **before** to do any code change one has to create a working branch. The following conventions are greatly encouraged:
+
+* `feature/my_outstanding_feature`
+* `issue/issue-number` (if no number is related to the issue please consider [opening an issue](https://gitlab.delpinux.fr/code/pastis/issues) and assign it to yourself)
+
+----
+## Tests and coverage
+
+### Running tests
+Tests are built automatically and are run typing
+> `make test`
+
+**all tests should be running correctly before any merge request**
+
+### Unit tests
+Unit tests are defined in the `tests` directory. New unit tests should be written or updated when needed.
+
+### Coverage
+Preferably, one should install [gcovr](http://www.gcovr.com) and build `pugs` specifying the `Coverage` build type
+> `cmake -DCMAKE_BUILD_TYPE=Coverage [...]`
+
+However coverage is computed at each `push` by the `gitlab-ci`.
+
+----
+## Coding
+
+### `packages` directory
+Do not make **any** change in the `packages` directory. This directory contains *third party libraries* that are are mandatory to minimal builds or `pugs`.
+
+For any changes in this directory
+(bug report, suggestion of new library, library upgrades or downgrades) **open an issue**.
+
+### Warnings
+Try to avoid publishing sources that generates compilation warnings.
+
+Also, avoid the use of `#warning` directives and prefer opening an issue.
+
+`C++` `[[deprecated]]` directive should also be avoid as much as possible.
+
+### Comments
+
+**Do not** comment deprecated code. It is `git`'s job to keep track of old versions.
+
+Avoid commenting functions bodies:
+
+* comments are likely to be deprecates
+* if the code is not clear by itself comments will not help
+
+`Doxygen` is to be used to comment functions in the header.
+
+### Code formatting
+
+`clang-format` is used in `pugs`, so install it and run
+> `make clang-format` before any commit
+
+A better solution is to configure your favored editor to perform formatting automatically. For instance `emacs` users should copy-past the following code to their `.emacs.el` init file.
+```lisp
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Clang-format
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; clang-format can be triggered using C-c C-f
+;; Create clang-format file using google style
+;; clang-format -style=google -dump-config > .clang-format
+(use-package clang-format
+  :ensure t
+  :bind (("C-c C-f" . clang-format-region))
+  )
+;; sets clang-format to be used instead of emacs indentation
+(defun clang-format-c-mode-common-hook ()
+  (fset 'c-indent-region 'clang-format-region)
+  (define-key c-mode-base-map (kbd "<tab>") 'clang-format-region)
+  )
+(add-hook 'c-mode-common-hook 'clang-format-c-mode-common-hook)
+;; sets clang-format to be run on save for the whole file
+(defun clang-format-buffer-on-save ()
+  "Add auto-save hook for clang-format-buffer-smart."
+  (add-hook 'before-save-hook 'clang-format-buffer nil t))
+(add-hook 'c-mode-common-hook 'clang-format-buffer-on-save)
+```