docs(README.md): add how to install and to use

Notice:
- On Google Chrome the request permission dialog will appear after click user gesture on some part of the page, specifically on the body element.
- On Firefox the request permission dialog will appear after mouseover user gesture on some part of the page, specifically on the body element.
docs(README.md): add browser supports notes

Go to this page to see if your browser is supporting Web Notification:
http://caniuse.com/notifications
docs(README.md): link to spec page
docs(README.md): add how to running tests
docs(README.md): add license details
Merge branch 'enhance-the-readme'