Include CONTRIBUTING.md (#4)

Shows up when making a pull request.

Author: vsuksi

CONTRIBUTING.md

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+# Contributions
+Contributors and maintainers are expected to abide by the [Bioconductor Code of Conduct](https://bioconductor.github.io/bioc_coc_multilingual/en-US.html). 
+
+When creating an issue, use a clear and descriptive title, detail the issue and context in a verbose manner and include code for demonstration. 
+
+We follow a git flow approach for changes. Fork and clone the repository. Create a new branch and develop against current Bioc-devel version. Document documentation changes using devtools::document(). Check and resolve errors and warnings from devtools::check() and BiocCheck::BiocCheck(). Make sure that your branch is synced with the target branch and create a pull request. Detail the changes and context in a verbose manner and tag one of the maintainers for review. Also check the automatic build reports in the pull request.