Update README to reference a cannonical control sheet & templates

[timgdavies]

At the moment, each new instance of the tool ends up 'forking' a recent control sheet, master template and answer template.

This means that the Google Apps Script used to manage surveys is not being well-versioned or managed. Attempts to use clasp to push scripts to new sheets hit problems, due to new levels of approval needed on scripts, which we seem to get around by copying existing sheets with the scripts attached.

However, it would be better if:

• There is a stable, and a development copy of the Control Sheet, in known locations for copies to be taken from
• This is a template answer sheet in a stable location
• There is a template survey sheet which demonstrates all the features of the survey tool in a stable location

These should then be linked from the README file, referenced as part of the setup process.

This work should also investigate options (either with clasp or alternatives) to better version control the google app script part of this project.

[timgdavies]

I've put together a template survey sheet here

Dropping this key 1pPfuytPdkWeEZCtSEQNfV5-dXg_qWoXqM4oolhSQ1gU onto the end of a working survey tool URL should support testing and checks on CSS layout to handle different question types.

(cc @aliblackwell )

[timgdavies]

I've set up a folder at https://drive.google.com/drive/folders/1RQy38MlSjoHmBQcRY-HKSOvHRlk7sr8r where the template scripts can eventually live.

For the time being I'm using GDB's Survey Tool Tests folder as a working copy to make updates.