create contribution guidelines

CONTRIBUTING.md

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+# Contributing & Guidelines
+
+In 🦸 Superhero UI we use [git-flow](https://danielkummer.github.io/git-flow-cheatsheet/) (you can check [this git-flow breakdown](https://gist.github.com/JamesMGreene/cdd0ac49f90c987e45ac) as well to familiarize yourself with the workflow). To contribute, please follow these rules:
+
+* A `develop` branch is created from `master`
+* A `release` branch is created from `develop`
+* Feature branches are created from `develop` and are prefixed with `feature/` (ex. `feature/show-token-balance`)
+* If there are conflicts and the newly created feature branch needs to be synced with `develop` - rebase your branch with `develop`, resolve any conflicts that may arise and push your changes to the remote branch.
+* When development is finished a pull request to `develop` is created. At least one person has to review the PR and when everything is fine the PR gets merged.
+* When a feature is complete it is merged into the `develop` branch
+* The `develop` branch and all the feature branches are deployed to the stage environment or feature preview environment by travis-ci automatically.
+* To make a new release create a release branch called release/vX.X.X, also bump the version number in package.json in this branch.
+* Create a PR to `master` which then also has to be accepted.
+* Create a `tag` for this version and push the `tag`.
+* Also merge back the changes (like the version bump) into `develop`.
+* The `master` branch has to be deployed to the production environment manually or via the automated deployment in travis-ci.
+* If an issue in master is detected a hotfix branch is created from `master`
+* Once the hotfix is complete it is merged to both `develop` and `master`
+
+
+All contributions are welcome :pray: