Merge pull request #16 from Print-one/docs/contributing

docs: 📝 write contributing.md

.env.example

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+API_URL=https://api.print.one
+API_KEY=test_abcdefghijklmnopqrstuvwxyz0123456789

CONTRIBUTING.md

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+# Contributing Guide
+
+## Setting up development environment
+
+### Prerequisites
+
+- Node.js
+- npm
+
+### Steps
+
+1. Clone the repository with `git clone https://github.com/Print-one/print-one-js.git`
+2. Enter the repository, and run `npm install` to install all dependencies.
+3. Run `npm run test` to run the tests. NOTE: This requires you to have followed the setting up test environment steps (see below).
+
+## Setting up test environment
+
+To setup the test environment, copy over the `.env.example` file to `.env` and fill in the required values.
+To then run the tests, run `npm run test`.
+
+## The "next" branch
+
+We have a branch called `next` that is used for features that are behind feature flags. All unreleased features / features still in development will be in this branch. If you want to contribute to a feature that is still in this state, make sure to branch off of `next` and submit a pull request to `next`.
+
+Next should also be kept in sync with `main` as much as possible, so that we can easily cherry-pick features into `main` when we are ready to release them.
+
+## Pull Requests
+
+Make sure when submitting a pull requests that your code passes the tests, and that you have written tests for any new features or bug fixes.