docs: added README

README.md

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+# jni-macro
+this is a simple procedural macro crate to automatically generate JNI-compatible extern functions
+
+it also wraps functions returning `Result<>`, making short-circuiting easy
+
+## usage
+just annotate your functions with
+
+```rust
+#[jni_macro::jni(package = "your.package.path", class = "ContainerClass")]
+fn your_function_name(arg: i32) -> Result<(), String> {
+	// your code here
+}
+```
+
+by specifying package and class, this crate will write an appropriate wrapper with the right function name. If inner function returns a `Result<>`, wrapper will also handle it. (currently just panics, soon will throw exceptions!)
+
+## examples
+the following function:
+```rust
+#[java_easy_jni::jni(package = "mp.code", class = "Client")]
+fn connect(env: JNIEnv, cacca: JString) -> Result<(), ConnectionError> {
+  let config = codemp::api::Config::new("asd".into(), "dsa".into());
+  tokio().block_on(codemp::Client::connect(config))?;
+  Ok(())
+}
+```
+
+gets turned into this couple of functions:
+```rust
+fn connect(env: JNIEnv, host: JString) -> Result<(), ConnectionError> {
+  let config = codemp::api::Config::new("mail@example.net".into(), "dont-use-this-password".into());
+  tokio::runtime::current().block_on(codemp::Client::connect(config))?;
+  Ok(())
+}
+
+#[no_mangle]
+pub extern "system" fn Java_mp_code_Client_connect<'local>(env: JNIEnv, host: JString) -> () {
+  match connect(env, cacca) {
+    Ok(x) => x,
+    Err(e) => {
+      $crate::panicking::panic_fmt($crate::const_format_args!("error in JNI!"));
+    }
+  }
+}
+```
+
+
+## status
+this crate is rather early and intended mostly to maintain [`codemp`](https://github.com/hexedtech/codemp) java bindings, however it's also quite small and only runs at comptime, so should be rather safe to use