Add Letters API to Python Docs
Add Letters API to Python Docs

Send Messages section:
- Amended arguments to be channel-specific and copied into each channel section from the end of the Send Messages section
- Added phone number argument to text message channel section

Get Status... sections:
- Updated statuses for Letters to created, sending, failed, technical failure
- Amended failed statuses to be channel appropriate

Corrections:
- Added note saying that we support up to six lines and a postcode and this will be put in the address block of the letter
- Changed all address line 2 for letters to be required rather than optional
- Changed all fake addresses to 123 High street
alphagov/notifications-python-client,alphagov/notifications-python-client