[pca] Recommended patches and PCA

[Martin Paul]

Hi Jonathan,

Thanks for the background information and the confirmation that "--minimal" 
works for you. I decided to make it official now, and have added documentation 
to the development release of PCA now.

> Without gushing too much we have found PCA too useful to not use. It
> simplifies and standardises everything for us.

You made my day! :)

Martin.