message: [skip ci] Add some contributing content
diff: +# Contributing to GUDHI
  +
  +First of all, thank you for the time you may take to contribute to GUDHI !
  +
  +# In case you have a question
  +
  +Please, check our [contact web page](https://gudhi.inria.fr/contact/).
  +
  +# In case you found an issue
  +
  +Please, first check [opened issues on GUDHI](https://github.com/GUDHI/gudhi-devel/issues).
  +
  +If the problem you are facing is not referenced, do not hesitate to open a [new issue](https://github.com/GUDHI/gudhi-devel/issues/new).
  +
  +This place is also a good place if you have some enhancement you want to propose for the GUDHI library.
  +There is a label **enhancement** in the [new issue](https://github.com/GUDHI/gudhi-devel/issues/new) page.
  +
  +# In case you want to contribute to GUDHI
  +
  +## You are not familiar with GitHub ?
  +
  +Please take some time to read our [how to use GitHub to contribute to GUDHI](/home/vincent/workspace/gudhi/gudhi-devel/for_dev/how_to_use_github_to_contribute_to_gudhi.md).
  +
  +## Something you want to improve in the documentation
  +
  +For C++ documentation, you can find it in the directories:
  +* *src/common/doc* for the main page and installation instructions
  +* *src/NAME_OF_THE_MODULE/doc* for the main page of a module
  +* *src/NAME_OF_THE_MODULE/include/gudhi* for the documentation generated from the code.
  +We use Doxygen to generate the code and you will be able to verify the result in CircleCI Doxygen target in the artifacts.
  +
  +For Python documentation, you can find it in the directories:
  +* *src/python/doc* for the main page, installation instructionsand for the main pages of the modules
  +* *src/python/gudhi/NAME_OF_THE_MODULE.pyx* for the documentation generated from the code.
  +We use Sphinx to generate the code and you will be able to verify the result in CircleCI Sphinx target in the artifacts.
  +
  +## Something you want to improve in the code
  +
  +Please first take some time to read our [code conventions](code_conventions.md)
  +
  