docs - Add helper-get-function-arity readme [skip ci] (#6532)

* Add README to babel-helper-get-function-arity

* Use javascript template

* Address code review

* Comment out ellipsis

packages/babel-helper-get-function-arity/README.md

@@ -1,5 +1,21 @@
 # @babel/helper-get-function-arity
 
+Function that returns the number of arguments that a function takes.
+* Examples of what is considered an argument can be found at [MDN Web Docs](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/length)
+
 ## Usage
 
-TODO
+```javascript
+import getFunctionArity from "@babel/helper-get-function-arity";
+
+function wrap(state, method, id, scope) {
+  // ...
+  if (!t.isFunction(method)) {
+    return false;
+  }
+
+  const argumentsLength = getFunctionArity(method);
+
+  // ...
+}
+```