Update readme

Author: rafgraph

README.md

@@ -22,25 +22,27 @@ $ npm install --save react-router-hash-link
 ```javascript
 // In YourComponent.js
 ...
-import { HashLink as Link } from 'react-router-hash-link';
+import { HashLink } from 'react-router-hash-link';
 ...
-// Use it just like a RRv4/5 <Link> (to can be a string or an object, see RRv4/5 api for details):
-<Link to="/some/path#with-hash-fragment">Link to Hash Fragment</Link>
+// Use it just like a RRv4/5 <Link> (to can be a string or an object, see RRv4/5 api for details)
+<HashLink to="/some/path#with-hash-fragment">Link to Hash Fragment</HashLink>
 ```
 
 
 ### `<NavHashLink>`
 ```javascript
 // In YourComponent.js
 ...
-import { NavHashLink as NavLink } from 'react-router-hash-link';
+import { NavHashLink } from 'react-router-hash-link';
 ...
-// Use it just like a RRv4/5 <NavLink> (see RRv4/5 api for details):
-<NavLink
+// Use it just like a RRv4/5 <NavLink> (see RRv4/5 api for details)
+// It will be active only if both the path and hash fragment match 
+<NavHashLink
   to="/some/path#with-hash-fragment"
   activeClassName="selected"
+  activeStyle={{ color: 'red' }}
   // etc...
->Link to Hash Fragment</NavLink>
+>Link to Hash Fragment</NavHashLink>
 ```
 
 ## Scrolling API
@@ -49,22 +51,39 @@ import { NavHashLink as NavLink } from 'react-router-hash-link';
 - 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>
+import { HashLink } from 'react-router-hash-link';
+<HashLink smooth to="/path#hash">Link to Hash Fragment</HashLink>
 ```
 
 ### `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
+import { HashLink } from 'react-router-hash-link';
+<HashLink
     to="/path#hash"
     scroll={el => el.scrollIntoView({ behavior: 'instant', block: 'end' })}
->Link to Hash Fragment</Link>
+>Link to Hash Fragment</HashLink>
 ```
 
-### Custom `Link`
+### Scroll to top of page
+- To scroll to the top of the page set the hash fragment to `#` (empty) or `#top`
+- This is inline with the [HTML spec](https://html.spec.whatwg.org/multipage/browsing-the-web.html#target-element), also see [MDN](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/a#Linking_to_an_element_on_the_same_page)
+```js
+import { HashLink } from 'react-router-hash-link';
+<HashLink to="/path#top">Link to Top of Page</HashLink>
+```
+
+### Scroll with offset
+- To scroll with offset use a custom scroll function, one way of doing this can be found [here](https://github.com/rafgraph/react-router-hash-link/issues/25#issuecomment-536688104)
+
+### `elementId: string`
+- Scroll to the element with matching id
+- Used instead of providing a hash fragment as part of the `to` prop, if both are present then the `elementId` will override the `to` prop's hash fragment
+- Note that it is generally recommended to use the `to` prop's hash fragment instead of the `elementId`
+
+
+## Custom `Link`
 
 The exported components are wrapped versions of the `Link` and `NavLink` exports of react-router-dom. In some cases you may need to provide a custom `Link` implementation.