Add contributing guide

CONTRIBUTING_EN.md

@@ -0,0 +1,73 @@
+# Contributing to Certimate
+
+Thank you for taking the time to improve Certimate! Below is a guide for submitting a PR (Pull Request) to the main Certimate repository.
+
+- [Contributing to Certimate](#contributing-to-certimate)
+  - [Prerequisites](#prerequisites)
+  - [Making Changes in the Go Code](#making-changes-in-the-go-code)
+  - [Making Changes in the Admin UI](#making-changes-in-the-admin-ui)
+
+## Prerequisites
+
+- Go 1.22+ (for Go code changes)
+- Node 20+ (for Admin UI changes)
+
+If you haven't done so already, you can fork the Certimate repository and clone your fork to work locally:
+
+```bash
+git clone https://github.com/your_username/certimate.git
+```
+
+> **Important:**
+> It is recommended to create a new branch from `main` for each bug fix or feature. If you plan to submit multiple PRs, ensure the changes are in separate branches for easier review and eventual merge.
+> Keep each PR focused on a single feature or fix.
+
+## Making Changes in the Go Code
+
+Once you have made changes to the Go code in Certimate, follow these steps to run the project:
+
+1. Navigate to the root directory.
+2. Start the service by running:
+   ```bash
+   go run main.go serve
+   ```
+
+This will start a web server at `http://localhost:8090` using the prebuilt Admin UI located in `ui/dist`.
+
+**Before submitting a PR to the main repository, consider:**
+
+- Adding unit or integration tests for your changes. Certimate uses Go’s standard `testing` package. You can run tests using the following command (while in the root project directory):
+
+  ```bash
+  go test ./...
+  ```
+
+## Making Changes in the Admin UI
+
+Certimate’s Admin UI is a single-page application (SPA) built using React and Vite.
+
+To start the Admin UI:
+
+1. Navigate to the `ui` project directory.
+2. Install the necessary dependencies by running:
+   ```bash
+   npm install
+   ```
+3. Start the Vite development server:
+   ```bash
+   npm run dev
+   ```
+
+You can now access the running Admin UI at `http://localhost:5173` in your browser.
+
+Since the Admin UI is a client-side application, you will also need to have the Certimate backend running. You can either manually run Certimate or use a prebuilt executable.
+
+Any changes you make in the Admin UI will be automatically reflected in the browser without requiring a page reload.
+
+After completing your changes, build the Admin UI so it can be embedded into the Go package:
+
+```bash
+npm run build
+```
+
+Once all steps are completed, you are ready to submit a PR to the main Certimate repository.