Document site.Params.mainSections usage

[cgalo5758]

I was pulling my hairs out trying to figure out why my homepage would only show content from only one section. I found out that the theme filters sections for the homepage using the mainSections parameter; This means that the homepage will by default show only the section with the most content.

[yriveiro]

I spent about a day on this as well, I would like to have a section for posts and another for books and display the book section on the main menu, but it seems that I can't have both :/

[cgalo5758]

This has already been fixed:

#1082