HTML uploads are only relevant for SilverStripe 3.1,
since they're disallowed by default from 3.2 onwards in
the File.allowed_extensions configuration already.
- Developers can add email templates via a configurable path.
- Content editors can select the email templates via a dropdown.
- Content editors can embed HTML content in emails.
- Content editors can preview HTML emails.
- Content editors can use field values (merge fields) in emails.
Split README.md and docs/en/index.md so that One is targeted towards users visiting the github repo, and the other for browsing a documentation repository.