docs: add README

README.md

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+# Shopee Coins Check-in
+
+Check-in to Shopee and get coins automatically 😀😀
+
+> **Warning**
+> This bot is only tested for Taiwan users, who uses [shopee.tw](https://shopee.tw/).
+
+The bot tries to login to user's Shopee account in the following order. If the previous one fails, the bot tries the next one until success.
+
+1. saved cookies if given
+2. username and password if both given
+
+If `--cookie` is set, the bot tries to login using cookies. Cookies are then updated if login succeeded.
+
+## Usage
+
+Login with username and password and save the cookies for the first time.
+
+```sh
+docker run -it -v /path/to/somewhere:/cookie \
+    hyperbola/shopee-coins-bot:v1 -u username -p password -c /cookie
+```
+
+> **Warning**
+> If this is your first login, you may need to authenticate by clicking the link in SMS message.
+
+Login with cookies only.
+
+```sh
+docker run -it -v /path/to/somewhere:/cookie hyperbola/shopee-coins-bot:v1 -c /cookie
+```
+
+Help message.
+
+```sh
+docker run -it hyperbola/shopee-coins-bot:v1 --help
+```
+
+Exit code:
+
+| Exit code | Description |
+| --------- | ----------- |
+| 0         | Success.    |
+| 1         | User has already received coins today. Returns 0 if `--force` is set. |
+| 2         | SMS authentication is needed but user refuses with `--no-sms`. |
+| 3         | Shopee requires the bot to solve a puzzle, but the bot is too stupid to play it. This occurs if the user login fails too may times. |
+| 4         | Operation timeout exceeded. |
+| 69        | Shopee rejects the login because of too much tries. |
+| 87        | Wrong password. |
+
+### Options
+
+Each option is optional.
+
+- `-u`, `--user`: shopee username
+- `-p`, `--pass`: shopee password
+- `-P`, `--path-to-pass`: shopee password file
+- `-c`, `--cookie`: cookie file
+- `-x`, `--no-sms`: do not login with SMS; default to `false`
+- `-f`, `--force`: no error if coins already received; default to `false`
+
+Shopee username is determined in the following order. The first meet is the final result.
+
+1. environment var `USERNAME`
+2. argument `--user`
+
+Shopee password is determined in the following order. The first meet is the final result.
+
+1. environment var `PASSWORD`
+2. argument `--pass`
+3. argument `--path-to-pass`