Add Buttons documentation.
Add Buttons documentation.

Summary: This CL adds the preliminary documentation for the Buttons component.

Test Plan: Look out for typos and inexactitudes.

Reviewers: featherless, ajsecord, #material_components_ios_owners

Reviewed By: featherless, ajsecord, #material_components_ios_owners

Subscribers: featherless

Projects: #material_components_ios_owners

Differential Revision: http://codereview.cc/D40

samnm/material-components-ios,sigito/material-components-ios,willlarche/material-components-ios,samnm/material-components-ios,randallli/material-components-ios,randallli/material-components-ios,samnm/material-components-ios,brianjmoore/material-components-ios,sigito/material-components-ios,randallli/material-components-ios,material-components/material-components-ios,sigito/material-components-ios,brianjmoore/material-components-ios,chriscox/material-components-ios,Eccelor/material-components-ios,chriscox/material-components-ios,chriscox/material-components-ios,willlarche/material-components-ios,Eccelor/material-components-ios,Eccelor/material-components-ios,brianjmoore/material-components-ios,samnm/material-components-ios,material-components/material-components-ios,chriscox/material-components-ios,material-components/material-components-ios,chriscox/material-components-ios,material-components/material-components-ios,brianjmoore/material-components-ios,sigito/material-components-ios,randallli/material-components-ios,sigito/material-components-ios,willlarche/material-components-ios,randallli/material-components-ios,Eccelor/material-components-ios,material-components/material-components-ios,Eccelor/material-components-ios,samnm/material-components-ios,randallli/material-components-ios,willlarche/material-components-ios,brianjmoore/material-components-ios,willlarche/material-components-ios