Update email_templates.md

Added warning at top - availble for Silver or higher
Merge branch 'patch-5' of https://github.com/bhammerquist/docs into bhammerquist-patch-5
Changed warning to info
Merge branch 'bhammerquist-patch-5' into develop
Merge branch 'develop' of https://github.com/sendgrid/docs into develop

Conflicts:
	source/Apps/email_templates.md