[doc] Add README.md

README.md

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+# constructor_array
+
+Module initialization functions for Rust (like __attribute__((constructor)) in C/C++) under no_std.
+
+
+After registering a constructor function, a function pointer pointing to it will be stored in the `ctor` section.
+
+
+When the program starts, it can call all initialization functions in the `ctor` section in order.
+
+## Usage
+
+```rust
+use constructor_array::register_ctor;
+#[register_ctor]
+fn hello_world() {
+    println!("Hello, world!");
+}
+
+static MAX_NUM: std::sync::atomic::AtomicUsize = std::sync::atomic::AtomicUsize::new(0);
+
+#[register_ctor]
+fn set_max_num() {
+    MAX_NUM.store(20, std::sync::atomic::Ordering::Relaxed);
+}
+
+fn main() {
+    constructor_array::invoke_ctors();
+    println!(
+        "MAX_NUM: {}",
+        MAX_NUM.load(std::sync::atomic::Ordering::Relaxed)
+    );
+}
+```
+
+## Notes 
+To avoid section-related symbols being optimized by the compiler, you need to add "-z nostart-stop-gc" to the compile flags (see <https://lld.llvm.org/ELF/start-stop-gc>).
+
+
+For example, in `.cargo/config.toml`:
+```toml
+[build]
+rustflags = ["-C", "link-arg=-z", "link-arg=nostart-stop-gc"]
+rustdocflags = ["-C", "link-arg=-z", "-C", "link-arg=nostart-stop-gc"]
+```