chore: return readme

README.md

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+# itconfig
+
+Easy build a configs from environment variables and use it in globally.
+
+We recommend you start with the [documentation].
+
+
+## Example usage
+
+```rust
+#[macro_use] extern crate itconfig;
+use dotenv::dotenv;
+
+config! {
+    DATABASE_URL: bool,
+    HOST: String => "127.0.0.1".to_string(),
+}
+
+fn main () {
+    dotenv().ok();
+    cfg::init();
+    assert_eq(cfg::HOST(), String::from("127.0.0.1");
+}
+```
+
+
+[documentation]: https://docs.rs/itconfig