armv7m: add README.md, fix #1705

[Detail]
Add README.md

[Verified Cases]
Build Pass: <py_engine_demo>
Test Pass:  <py_engine_demo>

hardware/arch/armv7m/README.md

-## Contents
+@page armv7m armv7m
 
-## Introduction
-**armv7m** arch files of armv7m
+# 概述
+**armv7m** 使AliOS Things支持armv7m架构的CPU(包括cortex-m3/cortex-m4/cortex-m7)。主要提供以下功能：
+- 中断管理与分发处理
+- 异常处理，包括crash处理与栈回溯
+- 任务切换，任务栈初始化等
+- 向量表定义，系统启动入口等
 
-### Features
-- armv7m system api
+## 版权信息
+> Apache license v2.0
 
-### Directories
-
-```sh
+## 目录结构
+```tree
 armv7m
-├─ gcc    # System adapter file of armv7m architecture for gcc
-├─ armcc  # System adapter file of armv7m architecture for armcc
-├─ iccarm # System adapter file of armv7m architecture for iccarm
-├─ panic  # panic adapter file of armv7m architecture
+|-- armcc
+|   |-- m3
+|   |-- m4
+|   `-- m7
+|-- common
+|   |-- backtrace.c
+|   |-- include
+|   |-- panic_armcc.S
+|   |-- panic_c.c
+|   |-- panic_gcc.sx
+|   |-- panic_iccarm.S
+|   |-- panic_mpu.c
+|   `-- port_c.c
+|-- gcc
+|   |-- m3
+|   |-- m4
+|   `-- m7
+|-- iccarm
+|   |-- m3
+|   |-- m4
+|   `-- m7
+|-- include
+|   |-- backtrace.h
+|   |-- k_compiler.h
+|   |-- k_types.h
+|   `-- port.h
+|-- package.yaml # AOS 编译配置文件
+`-- README.md
 ```
-### Dependencies
 
-## Reference
+## 依赖组件
+无
+
+# 常用配置
+无
+
+# 注意事项
+
+# FAQ