Add readme for functional tests.

[stacey-gammon]

Includes some tips and best practices to help with test stability.

[bmcconaghy]

LGTM, thanks so much for jumping on this. Very helpful.

[nreese]

great start for documenting better testing practices. Should contributing.md link to this readme. My only concern is having developers actually find this

lgtm
code review

[elasticmachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request

[elasticmachine]

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• continuous-integration/kibana-ci/pull-request

[stacey-gammon]

Should contributing.md link to this readme. My only concern is having developers actually find this

Great point Nathan. Now that I go digging though, it turns out there is already similar documentation on our developer docs here: https://www.elastic.co/guide/en/kibana/current/development-functional-tests.html#_functional_test_timing

So I guess I should have actually added this info there?

This is pretty confusing. Where are we supposed to be putting developer documentation? in md's in the repo, or in the external docs? Both are pretty bad options IMO, @timroes had some very cool ideas about better developer documentation.

For the short term, I'm going to abandon this, with the eventual goal of merging it with the current developer documentation. Otherwise we will now just have two sources of truth that will get out of sync, and an even more confusing experience of where documentation should actually go.

cc @archanid, in case you have any extra background, since you recently modified the functional test documentation.