Installation guide in wiki missing important part about CDK

[mario45211]

Hi,

Maybe I'm a little newbie in Github and I was unable to create PR to the wiki page, so I'm putting my improvement proposal to the Installation page in the wiki:

Installation

NPM

npm i ng-gallery @angular/cdk

// changes start

In case you're not using Angular Material in your project, remember to include CDK css styles into root stylesheet - it's required for lightbox component to operate correctly.

tl;dr put this in your styles.css file
@import '@angular/cdk/overlay-prebuilt.css';

More here: https://material.angular.io/cdk/overlay/overview#initial-setup

//changes end

I wrote information about standalone CDK installation when someone is trying use ng-gallery in project, which does not include Angular Material (as mine). For me that was a little tricky to figured out, why my lightbox preview doesn't work, so this may saves someone else time.

Cheers
Mario