Add note to docs about .ttc files

[dracos]

They're just compiled templates, and can be ignored if customising/adding your own templates.

[danbri]

Thanks! Perhaps also mention in http://code.fixmystreet.com/customising/ that http://www.template-toolkit.org/ is the place to learn more about the templating language (assuming it is:)

[andrewblack]

I had noticed them and realised from the date they were autogenerated. Are there any other files that are autogenerated that might confused people