chore: adjusted documentation (#9)

Author: 0xflotus

README.md

@@ -60,3 +60,26 @@ module.exports = [
   },
 ];
 ```
+
+#### Experimental BigInteger Support
+
+There is a similar rule to prove for BigInteger hexadecimal numbers. You can enable it similar to the `hex-under` rule. The default limit is 255.
+At the moment these found issues are not automatically fixable.
+
+```js
+// eslint.config.js
+
+const eslintPluginHexUnder = require("eslint-plugin-hex-under");
+
+module.exports = [
+  {
+    files: ["*.js"],
+    plugins: {
+      "hex-under": eslintPluginHexUnder,
+    },
+    rules: {
+      "hex-under/hex-under-bigint": "warn",
+    },
+  },
+];
+```