Update DOCUMENTATION.md

add new document via sms error

see alphagov/notifications-tech-docs#171

DOCUMENTATION.md

@@ -305,6 +305,7 @@ If the request is not successful, the client raises a `Notifications::Client::Re
 |`400`|`BadRequestError: Unsupported file type '(FILE TYPE)'. Supported types are: '(ALLOWED TYPES)'`|`BadRequestError`|Wrong file type. You can only upload .pdf, .csv, .txt, .doc, .docx, .xlsx, .rtf or .odt files|
 |`400`|`BadRequestError: File did not pass the virus scan`|`BadRequestError`|The file contains a virus|
 |`400`|`BadRequestError: Send files by email has not been set up - add contact details for your service at https://www.notifications.service.gov.uk/services/(SERVICE ID)/service-settings/send-files-by-email`|`BadRequestError`|See how to [add contact details to the file download page](#add-contact-details-to-the-file-download-page)|
+|`400`|`[{`<br>`"error": "BadRequestError",`<br>`"message": "Can only send a file by email"`<br>`}]`|Make sure you are using an email template|
 |`403`|`AuthError: Error: Your system clock must be accurate to within 30 seconds`|`AuthError`|Check your system clock|
 |`403`|`AuthError: Invalid token: API key not found`|`AuthError`|Use the correct API key. Refer to [API keys](#api-keys) for more information|
 |`429`|`RateLimitError: Exceeded rate limit for key type TEAM/TEST/LIVE of 3000 requests per 60 seconds`|`RateLimitError`|Refer to [API rate limits](#rate-limits) for more information|