spec: readme: rewrite documents in English, add license chapter

Signed-off-by: Zhouqi Jiang <luojia@hust.edu.cn>

sbi-spec/README.md

@@ -1,4 +1,4 @@
-# SBI 标准的 Rust 实现
+# `sbi-spec`: RISC-V SBI constants and structures in Rust
 
 [![CI](https://github.com/rustsbi/sbi-spec/actions/workflows/build.yml/badge.svg?branch=main)](https://github.com/rustsbi/sbi-spec/actions)
 [![Latest version](https://img.shields.io/crates/v/sbi-spec.svg)](https://crates.io/crates/sbi-spec)
@@ -6,36 +6,43 @@
 [![Documentation](https://docs.rs/sbi-spec/badge.svg)](https://docs.rs/sbi-spec)
 ![license](https://img.shields.io/github/license/rustsbi/sbi-spec)
 
-这个库用 Rust 实现了 [SBI 标准](https://github.com/riscv-non-isa/riscv-sbi-doc) 定义的常量和结构。
-
-2.0 标准各章节的实现情况：
-
-- §3
-  - [x] 常量
-  - [x] 结构
-- §4
-  - [x] 常量
-- §5
-  - [x] 常量
-- §6
-  - [x] 常量
-- §7
-  - [x] 常量
-- §8
-  - [x] 常量
-- §9
-  - [x] 常量
-- §10
-  - [x] 常量
-- §11
-  - [x] 常量
-- §12
-  - [x] 常量
-- §13
-  - [x] 常量
-- §14
-  - [x] 常量
-- §15
-  - [x] 常量
-- §16
-  - [x] 常量
+This library implements the constants and structures defined in the [SBI Specification](https://github.com/riscv-non-isa/riscv-sbi-doc) using Rust.
+
+## Supported RISC-V SBI extensions
+
+Implementation status of the 3.0 specification by chapters:
+
+| # | Title | Support state |
+|:--|:------|:--------------|
+| §3 | Binary Encoding | Constants, Structures | 
+| §4 | Base Extension | Constants |
+| §5 | Legacy Extensions | Constants |
+| §6 | Timer Extension | Constants |
+| §7 | IPI Extension | Constants |
+| §8 | RFENCE Extension | Constants |
+| §9 | Hart State Management Extension | Constants |
+| §10 | System Reset Extension | Constants |
+| §11 | Performance Monitoring Unit Extension | Constants |
+| §12 | Debug Console Extension | Constants |
+| §13 | System Suspend Extension | Constants |
+| §14 | CPPC Extension | Constants |
+| §15 | Nested Acceleration Extension | Constants |
+| §16 | Steal-time Accounting Extension | Constants |
+| §17 | Supervisor Software Events Extension | Constants |
+| §18 | Firmware Features Extension | Constants |
+| §19 | Debug Triggers Extension | Constants |
+| §20 | Message Proxy Extension | Constants |
+
+Although deprecated, legacy extensions are retained under `#[cfg(feature = "legacy")]` to ensure
+compatibility with older software.
+
+## License & Copyright
+
+This project is licensed under either of
+
+- MIT license ([LICENSE-MIT](LICENSE-MIT) or [http://opensource.org/licenses/MIT](http://opensource.org/licenses/MIT))
+- Mulan PSL v2 ([LICENSE-MULAN](LICENSE-MULAN) or [https://opensource.org/licenses/MulanPSL-2.0](https://opensource.org/licenses/MulanPSL-2.0))
+
+Documents from RISC-V SBI Specification are used in this project. These documents are (C) RISC-V Foundation under
+Creative Commons Attribution 4.0 International License (CC-BY 4.0). The full license text is available
+at https://creativecommons.org/licenses/by/4.0/.