Add description about dynamic linking to README

Since the dynamic linking was introduced in the previous commit, this
commit updates the README to describe how to use dynamic linking mode
and provides basic usage examples for illustration.

Author: DrXiao

README.md

@@ -74,10 +74,19 @@ To execute the snapshot test, install the packages below:
 $ sudo apt-get install graphviz jq
 ```
 
+Additionally, because `shecc` supports the dynamic linking mode for the Arm architecture,
+it needs to install the ARM GNU toolchain to obtain the ELF interpreter and other dependencies:
+```shell
+$ sudo apt-get install gcc-arm-linux-gnueabihf
+```
+Another approach is to manually download and install the toolchain from [ARM Developer website](https://developer.arm.com/downloads/-/arm-gnu-toolchain-downloads).
+
+Select "x86_64 Linux hosted cross toolchains" - "AArch32 GNU/Linux target with hard float (arm-none-linux-gnueabihf)" to download the toolchain.
+
 ## Build and Verify
 
 Configure which backend you want, `shecc` supports ARMv7-A and RV32IM backend:
-```
+```shell
 $ make config ARCH=arm
 # Target machine code switch to Arm
 
@@ -86,13 +95,29 @@ $ make config ARCH=riscv
 ```
 
 Run `make` and you should see this:
+```shell
+$ make
+  CC+LD	out/inliner
+  GEN	out/libc.inc
+  CC	out/src/main.o
+  LD	out/shecc
+  SHECC	out/shecc-stage1.elf
+  SHECC	out/shecc-stage2.elf
 ```
+
+Run `make LINK_MODE=dynamic` to use the dynamic linking mode and generate the dynamically linked compiler:
+```shell
+# If using the dynamic linking mode, you should add 'LINK_MODE=dynamic' for each 'make' command.
+$ make LINK_MODE=dynamic
   CC+LD	out/inliner
   GEN	out/libc.inc
   CC	out/src/main.o
   LD	out/shecc
   SHECC	out/shecc-stage1.elf
   SHECC	out/shecc-stage2.elf
+
+$ file out/shecc-stage2.elf
+out/shecc-stage2.elf: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux-armhf.so.3, not stripped
 ```
 
 For development builds with memory safety checks:
@@ -103,22 +128,32 @@ $ make check-sanitizer
 
 File `out/shecc` is the first stage compiler. Its usage:
 ```shell
-$ shecc [-o output] [+m] [--no-libc] [--dump-ir] <infile.c>
+$ shecc [-o output] [+m] [--no-libc] [--dump-ir] [--dynlink] <infile.c>
 ```
 
 Compiler options:
 - `-o` : Specify output file name (default: `out.elf`)
 - `+m` : Use hardware multiplication/division instructions (default: disabled)
 - `--no-libc` : Exclude embedded C library (default: embedded)
 - `--dump-ir` : Dump intermediate representation (IR)
+- `--dynlink` : Use dynamic linking (default: disabled)
 
-Example:
+Example 1: static linking mode
 ```shell
 $ out/shecc -o fib tests/fib.c
 $ chmod +x fib
 $ qemu-arm fib
 ```
 
+Example 2: dynamic linking mode
+
+Notice that `/usr/arm-linux-gnueabihf` is the ELF interpreter prefix. Since the path may be different if you manually install the ARM GNU toolchain instead of using `apt-get`, you should set the prefix to the actual path.
+```shell
+$ out/shecc --dynlink -o fib tests/fib.c
+$ chmod +x fib
+$ qemu-arm -L /usr/arm-linux-gnueabihf fib
+```
+
 ### IR Regression Tests
 
 To ensure the consistency of frontend (lexer, parser) behavior when working on it, the snapshot test is introduced.
@@ -142,6 +177,7 @@ use `update-snapshot` / `check-snapshot` instead.
 
 `shecc` comes with a comprehensive test suite (200+ test cases). To run the tests:
 ```shell
+# Add 'LINK_MODE=dynamic' if using the dynamic linking mode.
 $ make check          # Run all tests (stage 0 and stage 2)
 $ make check-stage0   # Test stage 0 compiler only
 $ make check-stage2   # Test stage 2 compiler only