[Slider] Added swift example to slider readme
[Slider] Added swift example to slider readme

Summary: ongoing work for: https://github.com/google/material-components-ios/issues/318

Reviewers: #mdc_ios_owners, ajsecord, featherless

Reviewed By: #mdc_ios_owners, ajsecord, featherless

Subscribers: ajsecord, featherless

Projects: #material_components_ios

Differential Revision: http://codereview.cc/D658

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