added useful info to README

README.md

@@ -4,3 +4,32 @@
 ```SHELL
 curl -sSL "https://git.openergy.ru/LUCKFOX/SIM800C/raw/branch/main/install.sh" | bash
 ```
+
+---
+
+## Usage (via systemd unit)
+
+- Start & enable on startup
+```SHELL
+sudo systemctl start sim800
+sudo systemctl enable sim800
+```
+
+- Stop
+```SHELL
+sudo systemctl stop sim800
+```
+
+- Restart
+```SHELL
+sudo systemctl restart sim800
+```
+
+---
+
+## Dependencies
+
+### Ubuntu
+```SHELL
+sudo apt install ppp
+```