Document approach to form submit buttons

[benjclark]

There was recently a discussion on Slack wrt whether to use <input type="submit"> or <button> for form submit buttons. It was agreed in the end that we should use <button>.

See #frontend 26th Sept 1:54PM: https://springernature.slack.com/archives/C0GDUEGMS/p1506430489000133

Someone needs to formalise this in the playbook as an agreed approach including rationale.