Update md file by adding a simple example

the example is copied form here https://github.com/final-form/react-final-form-hooks

docs/api/useField.md

@@ -36,3 +36,41 @@ An object that looks just like [`FieldProps`](../types/FieldProps), except witho
 `useField()` returns [`FieldRenderProps`](../types/FieldRenderProps). It will manage the rerendering of any component you use it in, i.e. the component will only rerender if the field state subscribed to via `useField()` changes.
 
 `useField()` is used internally inside [`<Field/>`](Field).
+
+
+## Example 
+
+```ts
+import { useForm, useField } from 'react-final-form-hooks'
+
+const MyForm = () => {
+  const { form, handleSubmit, values, pristine, submitting } = useForm({
+    onSubmit, // the function to call with your form values upon valid submit
+    validate // a record-level validation function to check all form values
+  })
+  const firstName = useField('firstName', form)
+  const lastName = useField('lastName', form)
+  return (
+    <form onSubmit={handleSubmit}>
+      <div>
+        <label>First Name</label>
+        <input {...firstName.input} placeholder="First Name" />
+        {firstName.meta.touched && firstName.meta.error && (
+          <span>{firstName.meta.error}</span>
+        )}
+      </div>
+      <div>
+        <label>Last Name</label>
+        <input {...lastName.input} placeholder="Last Name" />
+        {lastName.meta.touched && lastName.meta.error && (
+          <span>{lastName.meta.error}</span>
+        )}
+      </div>
+      <button type="submit" disabled={pristine || submitting}>
+        Submit
+      </button>
+    </form>
+  )
+}
+
+```