docs: introduce project and how to use

.env.example

@@ -0,0 +1,2 @@
+TOKEN = ''  # replace with bot token
+NOTIFY_USER_ID = ''  # replace with user/group id which will receive message

README.md

@@ -1,2 +1,53 @@
 # get-LAN-IP-telegram-bot-rs
-The Rust version of get-LAN-IP-telegram-bot
+The Rust version of [get-LAN-IP-telegram-bot](https://github.com/hms5232/get-LAN-IP-telegram-bot).
+
+## Background
+
+The original project is developed for Raspberry Pi. This saves me much time from IP checking.
+
+But, I'm not sure why the Python virtual environment can't get IP when RPi just booted. I resolved it with a stupid way: delay.
+
+This actually work. However, I need to keep waiting  even device had booted until message delivered.
+
+Now, I migrate this project to Rust, and it seems more fast and reliable in my case.
+
+## Usage
+
+### Config
+
+You can use environment variables or dotenv.
+
+#### Environment
+
+```shell
+export TOKEN='bot token'
+export NOTIFY_USER_ID='chat id'
+```
+
+#### dotenv
+
+```shell
+cp .env.example .env
+```
+
+then replace value for your bot and chat/user id.
+
+※ The `.env` file should be placed same or parent dir with executable binary file. 
+
+### Run on RPi
+
+Add the following line into `/etc/rc.local`:
+
+```shell
+(cd /path/to/binary && ./get-LAN-IP-telegram-bot-rs; cd -)&
+```
+
+for example:
+
+```shell
+(cd /home/pi/Documents/get-LAN-IP-telegram-bot-rs && ./target/release/get-LAN-IP-telegram-bot-rs; cd -)&
+```
+
+## LICENSE
+
+[MPL 2.0](LICENSE)