docs: add doc comments and update readme

.gitignore

@@ -2,3 +2,4 @@
 *.ez
 build
 erl_crash.dump
+src/example.gleam

README.md

@@ -1,22 +1,41 @@
 # gwt
 
+Encode and decode JWTs.
+
 [![Package Version](https://img.shields.io/hexpm/v/gwt)](https://hex.pm/packages/gwt)
 [![Hex Docs](https://img.shields.io/badge/hex-docs-ffaff3)](https://hexdocs.pm/gwt/)
 
-## Quick start
+## Quickstart
 
-```sh
-gleam run   # Run the project
-gleam test  # Run the tests
-gleam shell # Run an Erlang shell
+```gleam
+import gwt.{type Jwt, type Verified, type Unverified}
+
+pub fn main() {
+  let jwt = 
+    gwt.new()
+    |> gwt.set_subject("1234567890")
+    |> gwt.set_audience("0987654321")
+    |> gwt.set_not_before(1_704_043_160)
+    |> gwt.set_expiration(1_704_046_160)
+    |> gwt.set_jwt_id("2468")
+
+  let jwt_without_signature = gwt.to_string(jwt)
+  let jwt_with_signature = gwt.to_signed_string(jwt, gwt.HS256, "lucy")
+
+  let assert Ok(unverified_jwt) = 
+    jwt_without_signature
+    |> gwt.from_string()
+
+  let assert Ok(verified_jwt) = 
+    jwt_with_signature
+    |>gwt.from_signed_string("lucy")
+}
 ```
 
 ## Installation
 
-If available on Hex this package can be added to your Gleam project:
-
 ```sh
-gleam add gwt
+gleam add gwt 
 ```
 
-and its documentation can be found at <https://hexdocs.pm/gwt>.
+The documentation can be found at <https://hexdocs.pm/gwt>.