feat(Effects, Example): add createEffect

[timdeschryver]

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PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
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What is the current behavior?

Closes #1368

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[ngrxbot]

Preview docs changes for 47991c3 at https://previews.ngrx.io/pr1667-47991c3/

[ngrxbot]

Preview docs changes for 6671dbf at https://previews.ngrx.io/pr1667-6671dbf/

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[ngrxbot]

Preview docs changes for 36a4eeb at https://previews.ngrx.io/pr1667-36a4eeb/

[alex-okrushko]

Should it be Observable for the dispatch false? We shouldn't force it to be an Action for dispatch: false.

[timdeschryver]

I had it first as a non action but it seemed a bit weird to me while changing the example-app,
because the non-dispatching effects were all taps on the actions stream.

But now that you mention it, I do agree. It would make an effect harder to implement on a non-actions stream.

What do you think @brandonroberts ?

[brandonroberts]

I agree also. I thought the same, but I didn't mention it in the review 😞