docs ~ add customized assertion example

examples/pretty_assertion_custom.rs

@@ -0,0 +1,17 @@
+#[allow(unused_imports)]
+use pretty_assertions;
+
+macro_rules! assert_eq { ($($arg:tt)+) => ({
+    pretty_assertions::with_config_assert_eq!(
+        pretty_assertions::Config {
+            auto_label: true,
+            style: pretty_assertions::Color::Yellow.normal(),
+            prefix: "<=>",
+            prefix_left: "<<=", prefix_right: "=>>",
+            ..Default::default()
+            },
+        $($arg)+
+    )
+})}
+
+include!("standard_assertion.rs");