add README

README.md

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+# certificate-expire-check
+
+A simple Python utility to monitor SSL/TLS certificate expiration for a list of domains and send notifications via email.
+
+## Project Structure
+
+```
+project/
+├── conf/
+│   ├── mail.conf       # SMTP configuration
+│   └── domains.txt     # List of domains and optional ports
+├── main.py             # Entry-point script
+├── modules/
+│   ├── __init__.py
+│   ├── config.py       # Configuration loader
+│   ├── domains.py      # Domain list parser
+│   ├── checker.py      # Certificate expiration checker
+│   └── notifier.py     # SMTP notifier
+├── Makefile
+└── README.md
+```
+
+## Prerequisites
+
+* Python 3.9+
+* tzdata
+* make(optional)
+* Root or sudo privileges for auto installation and cron setup with Makefile
+
+## Configuration
+
+1. **SMTP Settings**: Edit `conf/mail.conf`:
+
+2. **Domain List**: Edit `conf/domains.txt`, one domain per line. Append `:port` to specify a custom port (default is 443):
+
+   ```
+   example.com:443
+   google.com
+   expired.badssl.com:8443
+   ```
+
+## Installation
+
+From the project root, run:
+
+```bash
+sudo make all
+```
+
+This will:
+
+1. Create a system user `certificate-expire-check` if it does not exist
+2. Copy project files to `/etc/certificate-expire-check`
+3. Set ownership to the `certificate-expire-check` user
+4. Install a cron job that runs every 10 days at midnight
+
+## Makefile Targets
+
+* **all**: Runs `user`, `install`, and `cron` in sequence
+* **user**: Creates the system user
+* **install**: Copies files and sets permissions
+* **cron**: Adds the following cron entry for the user:
+
+  ```cron
+  0 0 */10 * * /usr/bin/python3 /etc/certificate-expire-check/main.py >> /var/log/certificate-expire-check.log 2>&1
+  ```
+* **clean**: Removes the cron job, deletes the system user, and removes the installation directory
+
+## Usage
+
+* Logs are written to `/var/log/certificate-expire-check.log`.
+* Manually trigger the check with:
+
+  ```bash
+  sudo -u certificate-expire-check /usr/bin/python3 /etc/certificate-expire-check/main.py
+  ```