docs: added contribution doc

CONTRIBUTING.md

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+# Contributing Guidelines
+
+## Setup
+
+### Install git
+For instructions see https://git-scm.com/.
+
+### Fork the project
+Go to https://github.com/drageelr/manim-data-structures and click the "fork" button.
+
+### Clone your fork
+```bash
+git clone https://github.com/<your-username>/manim-data-structures.git
+```
+Once your fork is cloned, change the directory to enter the project folder:
+```bash
+cd manim-data-structures
+```
+
+### Add upstream repository
+```bash
+git remote add upstream https://github.com/drageelr/manim-data-structures.git
+```
+Now, `git remote -v` should show two remotes:
+* `origin`: Your forked repository.
+* `upstream`: The Manim Data Structures repository
+
+### Install dependencies
+* Install [Poetry](https://python-poetry.org/) by following the instructions on this [link](https://python-poetry.org/docs/master/#installing-with-the-official-installer).
+* Run `poetry install` inside the fork directory to install all dependencies. This command also creates a virtual environment which you can later enter by running `poetry shell` from within the forked directory.
+* Install pre-commit by running `poetry run pre-commit install`. This ensures that each commit is properly formatted against the linters `black`, `flake8` and `isort`.
+
+## Development
+
+### Fetch the latest code from upstream
+```bash
+git checkout dev
+git pull upstream dev
+git push origin dev
+```
+
+### Initiate a PR
+Once you have finalized your contribution, navigate to this [link](https://github.com/drageelr/manim-data-structures/pulls) to create a new pull request and submit it.
+
+### Closing note
+Once your PR is approved, it will be merged into the `dev` branch and eventually your contribution will make it through to the next release.
+
+Thanks for contributing 😁!