DOCS README needs updating

[raissanorth]

The README file needs some attention, as it

• still refers to a config.php file and

• currently provides no upgrade information.

[wernerkrauss]

Related, also the changelog is out of date.

Upgrade information missing is a pain, as all widgeds are unpublished when updating to SS4. Is there an easy way or should we simply copy over the items to the newly created Live table with plain SQL?

[robbieaverill]

@wernerkrauss related issue for you: #169

[JorisDebonnet]

The video explaining what Widgets actually are (on introduction.md), could also use an update... it shows SS 2 I believe, a version I've never even worked with. And that page also links to https://www.silverstripe.org/widgets which doesn't work.

(And so I remain unconvinced I should start using widgets :P )

[robbieaverill]

You should use content blocks if you can =)

I'm not sure whether we'd be prioritising new promo videos for widgets - I suspect we won't be, so the fix is probably to remove it from the SS3 and SS4 documentation