Update README.md

Author: Qiu233

README.md

@@ -1 +1,32 @@
-# binary
+# binary
+Lean 4 package for binary data serialization and deserialization, with name `binary` inspired by Haskell's `binary` package.
+
+# Usage
+Add to `lakefile.lean`:
+```
+require binary from git "https://github.com/Qiu233/binary"
+```
+
+In file:
+
+```
+import Binary
+
+open Binary Primitive.LE
+...
+```
+
+Or open `Primitive.BE` when you want to handle data in big endian.
+
+The recommended usage is
+
+```
+import Binary
+
+open Binary Primitive
+
+open LE in
+def serialize_xxx : Put := do
+  ...
+```
+