Update Buttons README.
Update Buttons README.

Summary: This CL updates the Buttons' README file with the feedback received.

Test Plan: Check for typo and valid Markdown in the touched file.

Reviewers: featherless, #material_components_ios_owners, ajsecord

Reviewed By: #material_components_ios_owners, ajsecord

Subscribers: featherless

Projects: #material_components_ios_owners

Differential Revision: http://codereview.cc/D71

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