Remove required flag from contentLabel propType in Modal

Change 'set' to 'sets' in Modal.spec
Update README required props section

README.md

@@ -27,10 +27,9 @@ To install the stable version you can use [npm](https://npmjs.org/) or [yarn](ht
 
 ## Usage
 
-The Modal object has two required props:
+The Modal object has one required prop:
 
 - `isOpen` to render its children.
-- `contentLabel` to improve accessibility, since `v1.6.0`.
 
 Example:
 
@@ -48,6 +47,8 @@ Example:
 </Modal>
 ```
 
+> Use the prop `contentLabel` which adds `aria-label` to the modal if there is no label text visible on the screen, otherwise specify the element including the label text using `aria-labelledby`
+
 ### App Element
 
 The app element allows you to specify the portion

specs/Modal.spec.js

@@ -94,7 +94,7 @@ describe('State', () => {
     expect(contentAttribute(modal, 'role')).toEqual('dialog');
   });
 
-  it('set aria-label based on the contentLabel prop', () => {
+  it('sets aria-label based on the contentLabel prop', () => {
     const child = 'I am a child of Modal, and he has sent me here...';
     const modal = renderModal({
       isOpen: true,

src/components/Modal.js

@@ -56,7 +56,7 @@ export default class Modal extends Component {
     parentSelector: PropTypes.func,
     aria: PropTypes.object,
     role: PropTypes.string,
-    contentLabel: PropTypes.string.isRequired
+    contentLabel: PropTypes.string
   };
   /* eslint-enable react/no-unused-prop-types */