Help text in model_definition_{X}.txt files

[ld-archer]

We now have quite a lot of transition model types to choose from, some that have specific requirements (such as the random effects term in mixed effects models), and some other specific quirks (e.g. RandomForest model can't have modifiers like scale() or factor() in the formula). A big help comment might be useful for us and anyone else who tries to add a new model.

Requirements:

• List of available model types in groups
• Requirements for each model type (type of outcome variable, whether mixed effects, how to specify random effects etc.)
• Specific issues that need mentioning (like RandomForest mentioned above)
• Very basic information about how to add a brand new model type (probably just point to important scripts and basic info on what each one does)
• More?

Not thinking this can replace proper documentation, but wouldn't take long and could be useful.

[ld-archer]

Maybe better as a README.txt in the transitions directory as there are multiple model_definitions_X files.