Add clear instructions about snippets to documentation

[ProLoser]

It has a few times come up that people didn't realize that they must include the original library's snippet code themselves in addition to using this plugin.

Until this changes, I believe we should improve the documentation to make this notice clearly visible.
Addresses angulartics/angulartics#270 and angulartics/angulartics#379

[j-brown]

👍 thanks for creating this. It will be a great help!

[ProLoser]

@j-brown we will still need people to submit pull-requests 😛