Make documentation about how to update spreadsheet without breaking it

[milanlakhani]

It's not too fragile but it should be good to have guidelines anyway
Also may be worth having docs about the yaml structure

[milanlakhani]

• There should be no hidden rows and columns in the spreadsheet!
• Everything (whether a Controller, Control Action, UCA, CC, LS etc) should have an identifier. The identifier should be listed before text, separated by a colon. Ideally there should be no other colons in the document. Of course identifiers should be unique.
• Every file should have columns and row headings on A and 1, except Losses and Hazards which just has headings in row 1, Losses in A, Hazards in C and Scenario Constraints in E with no gaps between rows.