Add contribution guide

CONTRIBUTING.md

+# How to contribute to GPDS
+
+## Git management
+
+### Branches
+
+* One stable & protected master branch
+* Feature branches for development following the pattern `[dev_type]/[dev_name]` (ie. `chore/explaining_how_to_merge`) where [dev_type] can be:
+	* fix (bug fixes)
+	* feat (new feature)
+	* style (style modification)
+ 	* refactor (code refactoring)
+ 	* chore (base maintenance such as version bump)
+
+### Commit & branches merge
+* Commit name should follow pattern `[dev_type]: [brief description of the commit, lower than 50 characters]. [JIRA-KEY]`
+* All branches must be merged via a `merge request` (MR)
+* Merge requests should be created at the time of the branch creation in order to allow reviewer to comment and follow the developments, specify the `WIP` tag in the MR name (go further: *[feature highlight WIP](https://about.gitlab.com/2016/01/08/feature-highlight-wip/)*).  
+	Example:
+	```
+	git checkout -b chore/explaining_how_to_merge
+	git push --set-upstram origin chore/explaining_how_to_merge
+	```
+	Returns a link for creating the merge request easily: 
+	```
+	Total 0 (delta 0), reused 0 (delta 0)
+	remote:
+	remote: To create a merge request for chore/explaining_how_to_merge, visit:
+	remote:   https://forgemia.inra.fr/urgi-is/gpds/merge_requests/new?merge_request%5Bsource_branch%5D=chore/explaining_how_to_merge
+	remote:
+	To forgemia.inra.fr:urgi-is/gpds.git
+	* [new branch]      chore/explaining_how_to_merge -> chore/explaining_how_to_merge
+	La branche 'chore/explaining_how_to_merge' est paramétrée pour suivre la branche distante 'chore/explaining_how_to_merge' depuis 'origin'.
+	```
+
+* A `git rebase` is strongly recommanded before merging a MR
+	* [Git rebase official documentation](https://git-scm.com/book/en/v2/Git-Branching-Rebasing)
+	* [How to keep a clean history](https://about.gitlab.com/2018/06/07/keeping-git-commit-history-clean/)
+* Merge requests should be reviewed by at least 2 colleagues
+* Continuous Integration is launched automatically by Gitlab on each commit push or merge request creation.
+
+## Data management
+* Git LFS feature is enable on this project in order to store JSON test data
+* Another dedicated Git LFS project (internal only) will be created to handle all private + public JSON files
+* TODO: refer to the good Git LFS project in the CI settings for indexing the relevant data into the relevant Elasticsearch indices/instances
+* the JSON files generation will be handled by an external ET (extract/transform) tool.
+
+## Development environment
+* Look at the [README.md](README.md) for installation and execution instructions.
+* Recommanded IDE is [Intellij IDEA](https://www.jetbrains.com/idea/)
+* Use linting to apply code standards within the team:
+	* Use `ng lint` (for frontend code only)
+	* Use [Checkstyle](https://checkstyle.org/) and [PMD](https://pmd.github.io/) (TODO: implement) for backend (+frontend?) code
+* All runtime variables should be externalized from the code in order to facilitate the CI management (database host/port, application name, public URL, JSON location...) and the adoption by partners