chore(docs): add contributing guide

CONTRIBUTING.md

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+# Contributing
+Thank you for your interest in contributing to the project! 
+
+Since this project is mostly done by myself (with my poor coding skills and a lot of internet searching & using LLM sometimes), I am really appreciate to have your help here. <3
+
+## Table of Contents
+- [Ways to Contribute](#ways-to-contribute)
+- [Opening Issues](#opening-issues)
+- [Development](#development)
+    - [Development Environment](#development-environment)
+    - [Code Style](#code-style)
+- [Commits and Pull Requests](#commits-and-pull-requests)
+    - [Commits](#commits)
+    - [Pull Requests](#pull-requests)
+- [Code of Conduct](#code-of-conduct)
+
+All types of contributions are encouraged and valued. And here are some guidelines to help you get started:
+
+## Ways to Contribute
+If you like the project, but just don't have time to contribute, that's fine. :P
+
+There are other easy ways to support the project and show your appreciation, which I would also be very happy about:
+ - Star the project
+ - Send posts about it
+ - Refer this project in your project's readme
+ - Consider donating to the project (see [Donations](#donations) section below)
+
+## Opening Issues
+- Before opening a new issue, please check the [existing issues](https://github.com/GrassBlock1/mercury/issues) to avoid duplicates
+- There isn't any issue templates (yet), but please make sure you provide as much detail as possible about the issue including:
+   - Steps to reproduce
+   - Expected behavior
+   - Actual behavior
+   - Environment details (browser, OS, etc.)
+- For feature requests, describe the feature and why you think it would be useful
+
+## Development
+### Development Environment
+This project is built using Astro, requiring a Node.js (v22.15.1 (lts) is recommended) environment.
+
+You can set up your development environment by following these steps:
+1. Set up `nvm` according to the [nvm README.md](https://github.com/nvm-sh/nvm) (for fish users, please check [nvm.fish](https://github.com/jorgebucaran/nvm.fish))
+2. Install Node.js version:
+   ```bash
+   nvm install lts
+   ```
+3. The `pnpm` is used in this project, to install it (be sure to be in the nvm environment):
+   ```bash
+   npm install -g pnpm
+   ```
+4. And then clone the repository:
+   ```bash
+   git clone https://github.com/GrassBlock1/mercury.git
+   ```
+   Or if you are in a network that doesn't allow you to access GitHub, you can also try:
+   ```bash
+   git clone https://git.gb0.dev/gb/mercury
+   ```
+5. Change to the project directory and install dependencies:
+   ```bash
+   cd mercury && pnpm install
+   ```
+
+You are all set! Now you can start developing. To start the development server:
+```bash
+pnpm dev
+```
+This will start a local server at `http://localhost:4321` and watch for changes in the source files.
+### Code Style
+This project has no style formatting tool like `prettier` (yet),but make sure you follow existing conventions.
+
+- Use lower `camelCase` for Javascript constants and variables.
+- Use 2 spaces for indents in code
+- Follow existing patterns in the codebase
+## Commits and Pull Requests
+### Commits
+Now you already made changes to the code and want to commit it. please:
+
+- Make atomic commits (one feature/fix per commit)
+- Follow [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/) `(fix|chore|feat|refactor|revert): short description` styled message. 
+  - Format: `type(scope,optional): description`
+  - Types: `fix, feat, chore, docs, style, refactor, test, revert`
+  - Example: `feat(auth): add Google login option`
+
+### Pull Requests
+- Create a new properly-named branch for your changes
+- Make sure local tests (`astro build`) pass before submitting
+- Link any related issues in the PR description (if any)
+- Provide a clear description of the changes
+
+## Code of Conduct
+Please be respectful and considerate of others when contributing. I want to maintain a welcoming and inclusive environment for everyone.
+
+## Donations
+If you'd like to support the project financially, you can [support me on ko-fi](https://ko-fi.com/grassblock).
+
+Your support helps me to keep the project alive and improve it.
+
+Thank you for contributing! 💜