docs: add crate documentation

Cargo.toml

@@ -16,4 +16,5 @@ flate2 = "1.0.25"
 image = { version = "0.24.5", default-features = false, features = ["png"] }
 
 [features]
+# re-exports png crate
 png = []

src/lib.rs

@@ -1,3 +1,51 @@
+/*!
+apng is animated png encoder for Rust, and made in pure Rust.
+
+<img src="https://raw.githubusercontent.com/poccariswet/apng/master/examples/_rust_logo/out.png" width="250">
+
+# Example
+
+```rust
+fn main() {
+    let files = vec![
+        "rust_logo1.png",
+        "rust_logo2.png",
+        "rust_logo3.png",
+        "rust_logo4.png",
+        "rust_logo5.png",
+        "rust_logo6.png",
+    ];
+
+    let mut png_images: Vec<PNGImage> = Vec::new();
+    for f in files.iter() {
+        png_images.push(apng::load_png(f).unwrap());
+    }
+
+    let path = Path::new(r"sample/out.png");
+    let mut out = BufWriter::new(File::create(path).unwrap());
+
+    let config = apng::create_config(&png_images, None).unwrap();
+    let mut encoder = Encoder::new(&mut out, config).unwrap();
+    let frame = Frame {
+        delay_num: Some(1),
+        delay_den: Some(2),
+        ..Default::default()
+    };
+
+    match encoder.encode_all(png_images, Some(&frame)) {
+        Ok(_n) => println!("success"),
+        Err(err) => eprintln!("{}", err),
+    }
+}
+```
+
+
+# Feature Flags
+
+- `png`: re-exports the types from `png` crate
+
+*/
+
 mod apng;
 pub mod errors;
 mod png;