docs: add README for pim CLI tool documentation

README.md

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+<div align="center">
+<samp>
+
+# pim
+
+## a lightweight and extensible CLI tool for converting between various CSS-related units (e.g., "px", "em", "rem").
+
+</samp>
+</div>
+
+![Rust](https://img.shields.io/badge/Rust-1.72.0-orange)
+![License](https://img.shields.io/badge/license-MIT-blue.svg)
+
+---
+
+## Installation
+
+### Build from Source
+
+1. **Clone the repository**:
+   ```bash
+   git clone https://github.com/airRnot1106/pim.git
+   cd pim
+   ```
+
+2. **Build the binary**:
+   ```bash
+   cargo build --release
+   ```
+
+3. **Run the tool**:
+   ```bash
+   ./target/release/pim --help
+   ```
+
+### Using `nix`
+
+If you have `nix` installed:
+```bash
+nix run . -- --help
+```
+
+---
+
+## Usage
+
+### Basic Usage
+
+To convert `24px` to other unit:
+```bash
+pim 24 px
+```
+
+**Output**:
+```
+px: 24px
+em: 1.5em
+rem: 1.5rem
+```
+
+### Custom Root Font Size
+
+Specify a custom root font size using the `-r` option:
+```bash
+pim 24 px -r 10
+```
+
+**Output**:
+```
+px: 24px
+em: 2.4em
+rem: 2.4rem
+```
+
+## Tests
+
+Run all tests:
+```bash
+cargo test
+```
+
+---
+
+## License
+
+`pim` is licensed under the [MIT License](https://github.com/airRnot1106/pim/blob/main/LICENSE).