Update DOCUMENTATION.md

[leohemsted]

add new document via sms error

see https://github.com/alphagov/notifications-tech-docs/pull/171

What problem does the pull request solve?

Checklist

• [x] I’ve used the pull request template
• [ ] I’ve written unit tests for these changes
• [ ] I’ve update the documentation (in DOCUMENTATION.md)
• [ ] I’ve bumped the version number
  • [ ] in src/main/resources/application.properties
  • [ ] in pom.xml