Add example of using hooks

Adding a basic example using React Hooks in the Overview page.

docs/overview.md

@@ -160,6 +160,68 @@ const Basic = () => (
   </div>
 );
 
+export default Basic;
+```
+## Basic example using Hooks
+
+```jsx
+
+import { useFormik } from 'formik'
+
+const Basic = () => {
+  const formik = useFormik({
+    initialValues: {
+      email: '',
+      password: ''
+    },
+    validate: values => {
+      const errors = {};
+      if (!values.email) {
+        errors.email = 'Required';
+      } else if (
+        !/^[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,}$/i.test(values.email)
+      ) {
+        errors.email = 'Invalid email address';
+      }
+      return errors;
+    },
+    onSubmit: (values, { setSubmitting }) => {
+      setTimeout(() => {
+        alert(JSON.stringify(values, null, 2));
+        setSubmitting(false);
+      }, 400);
+    }
+  });
+
+  const { values, errors, touched, handleChange, handleBlur, handleSubmit, isSubmitting } = formik;
+  return (
+    <div>
+      <h1>Anywhere in your app!</h1>
+      <form onSubmit={handleSubmit}>
+        <input
+          type="email"
+          name="email"
+          onChange={handleChange}
+          onBlur={handleBlur}
+          value={values.email}
+        />
+        {errors.email && touched.email && errors.email}
+        <input
+          type="password"
+          name="password"
+          onChange={handleChange}
+          onBlur={handleBlur}
+          value={values.password}
+        />
+        {errors.password && touched.password && errors.password}
+        <button type="submit" disabled={isSubmitting}>
+          Submit
+        </button>
+      </form>
+    </div>
+  );
+}
+
 export default Basic;
 ```