Create CONTRIBUTING.md

Author: nbashir97

.github/CONTRIBUTING.md

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+# Contributing
+
+## Request for changes/ Pull Requests
+You first need to create a fork of the [effectsize][effectsize] repository to commit your changes to it. Methods to fork a repository can be found in the [GitHub Documentation][fork].
+
+Then add your fork as a local project:
+
+```sh
+# Using HTTPS
+git clone https://github.com/nbashir97/effectsize.git
+
+# Using SSH
+git clone git@github.com:nbashir97/effectsize
+```
+
+> [Which remote URL should be used ?][remote]
+
+Then, go to your local folder
+
+```sh
+cd effectsize
+```
+
+Add git remote controls :
+
+```sh
+# Using HTTPS
+git remote add fork https://github.com/YOUR-USERNAME/effectsize.git
+git remote add upstream https://github.com/nbashir97/effectsize.git
+
+
+# Using SSH
+git remote add fork git@github.com:YOUR-USERNAME/effectsize.git
+git remote add upstream git@github.com/nbashir97/effectsize.git
+```
+
+You can now verify that you have your two git remotes:
+
+```sh
+git remote -v
+```
+
+## Receive remote updates
+In view of staying up to date with the central repository :
+
+```sh
+git pull upstream master
+```
+
+## Choose a base branch
+Before starting development, you need to know which branch to base your modifications/additions on. When in doubt, use master.
+
+| Type of change                |           | Branches              |
+| :------------------           |:---------:| ---------------------:|
+| Documentation                 |           | `master`              |
+| Bug fixes                     |           | `master`              |
+| New features                  |           | `master`              |
+| New issues models             |           | `YOUR-USERNAME:patch` |
+
+```sh
+# Switch to the desired branch
+git switch master
+
+# Pull down any upstream changes
+git pull
+
+# Create a new branch to work on
+git switch --create patch/1234-name-issue
+```
+
+Commit your changes, then push the branch to your fork with `git push -u fork` and open a pull request on [the effectsize repository][effectsize] following the template provided.
+
+[effectsize]: https://github.com/nbashir97/effectsize
+[fork]: https://docs.github.com/en/get-started/quickstart/fork-a-repo
+[remote]: https://docs.github.com/en/get-started/getting-started-with-git/about-remote-repositories