docs(data): add ng add installation

[timdeschryver]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

I think this is the last step in order to close #1554

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[ngrxbot]

Preview docs changes for aba0a5c at https://previews.ngrx.io/pr1799-aba0a5c/

[ngrxbot]

Preview docs changes for 06af67e at https://previews.ngrx.io/pr1799-06af67e/

[johnpapa]

this is awesome!

Where should I make doc PRs - as I see a few tweaks I would like to suggest?

[brandonroberts]

The docs files are here https://github.com/ngrx/platform/tree/master/projects/ngrx.io/content/guide/data

[johnpapa]

ok - one example idiosyncratic - I think we should use a more widely known word :)

I'll do a run through later.

also - i'd like to update my labs with this, once it's ready to go. What's the timing for making it go live?

[brandonroberts]

It's ready to test with if you want to try out the latest beta, currently 8.0.0-beta.1. I don't foresee many changes to it other than docs