feat: add docs for StringAdapter::new() (#4)

src/lib.rs

diff --git a/src/lib.rs b/src/lib.rs
index a1001ee..baba7ca 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -28,6 +28,33 @@
 }
 
 impl StringAdapter {
+    /// # Examples
+    ///
+    ///```rust
+    /// # use casbin::{CoreApi, DefaultModel, Enforcer, Result};
+    /// # use string_adapter::StringAdapter;
+    /// # #[tokio::main]
+    /// # async fn main() -> Result<()> {
+    /// # let m = DefaultModel::from_file("examples/rbac_model.conf").await?;
+    /// let a = StringAdapter::new(
+    ///     r#"
+    ///     p, alice, data1, read
+    ///     p, bob, data2, write
+    ///     p, data2_admin, data2, read
+    ///     p, data2_admin, data2, write
+    ///     g, alice, data2_admin
+    ///     "#,
+    /// );
+    /// let e = Enforcer::new(m, a).await?;
+    ///
+    /// assert_eq!(true, e.enforce(("alice", "data1", "read"))?);
+    /// assert_eq!(true, e.enforce(("alice", "data2", "read"))?);
+    /// assert_eq!(true, e.enforce(("bob", "data2", "write"))?);
+    /// assert_eq!(false, e.enforce(("bob", "data1", "write"))?);
+    /// #   Ok(())
+    /// # }
+    ///
+    /// ```
     pub fn new(s: impl ToString) -> Self {
         Self {
             policy: s.to_string(),