Added edit link tag helper with readme description too

[prjseal]

This is a tag helper element which renders an edit link on the front end only if the current user is logged into umbraco and has access to the content section.

[prjseal]

@warrenbuckley. PR updated. I have simplified the edit link code a lot, taking out most of the styling options and leaving a simple bool to say if they want to use default styles or not.

I couldn't find a way to get the edit text if used like this:

<our-edit-link content-id="@(Model.Id)">
    Get this text
</our-edit-link>

So I just made it an attribute instead called text and this value defaults to "Edit"

I've updated the readme too, with a few more examples and notes on the attributes you can change.