Add documentation regarding field trial test config for in-product help

The //components/feature_engagement component is fully configured
through Chrome Variations, so it is required to also set up the
expected launch configuration in the field trial testing configuration.
This is the case for all Chrome Variations experiments, but since the
rest of the documentation for feature engagement is very specific, it
is easy to forget this part while following the guide.

This CL therefore adds it to the required steps and links to the
relevant configuration and adds a sample configuration so developers
can get a feel for what they are supposed to do.

BUG=None

Change-Id: I7b50683d803f2d1d646a2632b08dd1e8050408e4
Reviewed-on: https://chromium-review.googlesource.com/707306
Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#507475}

• components/feature_engagement/README.md[diff]