docs: add doc on how to contribute to this repo

CONTRIBUTING.md

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+# Contributing
+
+Feel free to dive in! [Open](https://github.com/PaulRBerg/create-eth-app/issues/new/choose) an issue,
+[start](https://github.com/PaulRBerg/create-eth-app/discussions/new/choose) a discussion or submit a PR.
+
+Contributions to `create-react-app` are welcome by anyone interested in adding support for more frameworks or DEFI templates, writing more tests, improving readability, or improving the documentation.
+
+## Pre Requisites
+
+You will need the following software on your machine:
+
+- [Git](https://git-scm.com/downloads)
+- [Node.Js](https://nodejs.org/en/download/)
+- [Yarn](https://yarnpkg.com/)
+
+## Set Up
+
+1. Fork this repository by visiting https://github.com/PaulRBerg/create-eth-app/fork
+
+2. Once the fork is created, head to the forked repo. For example, [this](https://github.com/robin-thomas/create-eth-app) is a fork of the original repo.
+
+3. Go to `Settings` tab of the forked repo. Under `Security` subheader, click on `Secrets and variables`, and click on `Actions`.
+
+4. Click on the `New repository secret` button, and add the following secrets:
+   - `GH_OAUTH_TOKEN`: Your fine-grained personal access token
+
+5. You can create a token by visiting: https://github.com/settings/personal-access-tokens/new
+  - Enter the token name as: `create-eth-app`
+  - Make sure `Only select repositories` option is selected, and choose your forked repo.
+  - Enable the `Secrets` permission under `Repository permissions` section, by choosing `Read-only`
+  - Use the token generated in the previous step as the value for `GH_OAUTH_TOKEN` secret.
+
+6. Clone the forked repo to your local machine
+
+7. Then, inside the project's directory, run this to install the Node.js dependencies:
+```bash
+$ yarn install --frozen-lockfile
+```
+
+Now you can start making changes.
+