Revise documentation (#37)

* Add User-Agent documentation to README

* Add User-Agent header value

* Revise documentation

README and serverless/DEVELOPER_README have been edited and now stress one-click deployment as the best way to deploy the connector.

Additionally, various grammatical, spelling, whitespace, and markdown formatting issues have been fixed. British English has been favoured. An outdated image was also changed.

GETTING_STARTED has been removed and its verification section has been moved to README.