Create CONTRIBUTING.md

Author: BDonnot

CONTRIBUTING.md

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+# Contribution
+
+We welcome contributions from everyone. 
+
+If you want to contribute, a good starting point is to get in touch with us through:
+- github (discussion, issues or pull-request)
+- the project [discord](https://discord.gg/cYsYrPT)
+- mail (see current corresponding author from the grid2op package on pypi here https://pypi.org/project/Grid2Op/) 
+
+Contribution can take different forms:
+
+- reporting bugs
+- improving the documentation
+- improving the code examples (notebooks or example in the doc)
+- fixing some reported issues (for example at https://github.com/rte-france/Grid2Op/issues )
+- adding a new functionality to grid2op (or increase its speed)
+- extend grid2op 
+
+# What to do ?
+
+For smaller changes (including, but not limited to the reporting of a bug or a contribution to the explanotory notebooks or the documentations)
+a simple "pull request" with your modifications by detailing what you had in mind and the  goal of your changes.
+
+In case of a major change (or if you have a doubt on what is "a small change"), please open an issue first
+to discuss what you would like to change and then follow as closely as possible the guidelines below. This is to ensure
+first that no other contribution is this direction is being currently made but also to make sure that it 
+does not conflicts with some core ideas of the project.
+
+# Guidelines for contributing to the codebase
+
+For larger contributions, you can follow the given :
+
+1. fork the repository located at <https://github.com/rte-france/Grid2Op>
+2. synch your fork with the "latest developement branch of grid2op". For example, if the latest grid2op release
+   on pypi is `1.10.3` you need to synch your repo with the branch named `dev_1.10.4` or `dev_1.11.0` (if
+   the branch `dev_1.10.4` does not exist). It will be the highest number in the branches `dev_*` on
+   grid2op official github repository.
+3. implement your functionality / code your modifications, add documentation or any kind of contribution
+4. make sure to add tests and documentation if applicable
+5. once it is developed, synch your repo with the last development branch again (see point 2 above) and
+   make sure to solve any possible conflicts
+6. write a pull request and make sure to target the right branch (the "last development branch")
+
+# When will it be merged ?
+
+A contribution will be merged once approved by the maintainers (this is why it is recommended to first 
+get in touch with the team)
+
+Code in the contribution should pass all the current tests, have some dedicated tests for the new feature (if applicable)
+and documentation (if applicable).
+
+New contributions should respect the "**Code of Conduct**" of grid2op.