Add scrolling api to readme

Author: rafgraph

README.md

@@ -14,7 +14,7 @@ $ yarn add react-router-hash-link
 $ npm install --save react-router-hash-link
 ```
 
-## `<HashLink>`
+### `<HashLink>`
 ```javascript
 // In YourComponent.js
 ...
@@ -25,7 +25,7 @@ import { HashLink as Link } from 'react-router-hash-link';
 ```
 
 
-## `<NavHashLink>`
+### `<NavHashLink>`
 ```javascript
 // In YourComponent.js
 ...
@@ -35,5 +35,27 @@ import { NavHashLink as NavLink } from 'react-router-hash-link';
 <NavLink
   to="/some/path#with-hash-fragment"
   activeClassName="selected"
+  // etc...
 >Link to Hash Fragment</NavLink>
 ```
+
+### Scrolling API
+`smooth: boolean`
+- Smooth scroll to the element
+- React Router Hash Link uses the native Element method `element.scrollIntoView()` for scrolling, and when the `smooth` prop is present it will call it with the smooth option, `element.scrollIntoView({ behavior: 'smooth' })`
+- Note that not all browsers have implemented options for `scrollIntoView` - see [MDN](https://developer.mozilla.org/en-US/docs/Web/API/Element/scrollIntoView) and [Can I Use](https://caniuse.com/#feat=scrollintoview) - there is also a browser [polyfill for smooth scrolling](https://github.com/iamdustan/smoothscroll) which you can install separately so `smooth` will work in all browsers
+```js
+import { HashLink as Link } from 'react-router-hash-link';
+<Link smooth to="/path#hash">Link to Hash Fragment</Link>
+```
+
+`scroll: function`
+- Custom scroll function called with the element to scroll to, e.g. `const myScrollFn = element => {...}`
+- This allows you to do things like scroll with offset, use a specific smooth scrolling library, or pass in your own options to `scrollIntoView`
+```js
+import { HashLink as Link } from 'react-router-hash-link';
+<Link
+    to="/path#hash"
+    scroll={el => el.scrollIntoView({ behavior: 'instant', block: 'end' })}
+>Link to Hash Fragment</Link>
+```