Wiki build instructions updated for product sets

[Elvis Stansvik]

Hi all,

I took the liberty of updating

http://community.kde.org/Calligra/Building#Building_selected_applications

to include info about product sets.

Would be nice if someone (Friedrich?) could take a short look to make
sure it's correct. Was a while since I built Calligra :)

Also, remember to update the wiki if you make any significant changes
to the product sets.

Cheers,
Elvis