Material: md-select: Document md-optgroup

[epelc]

Currently md-optgroup for md-select only has an example here. It should have a page listing the attributes and what they do(currently only label) like the other directives.

It's also not mentioned on the md-select api page while md-option is.

https://material.angularjs.org/latest/api/directive/mdSelect

[ThomasBurleson]

This issue is closed as part of our ‘Surge Focus on Material 2' efforts.
For details, see our forum posting @ http://bit.ly/1UhZyWs.

[hayzey]

How not to manage a project 101.