Add warning against an excessive number of partitions

[lolmaus]

Hi!

Our tests started failing with:

not ok 2212 Chrome - error
    ---
        message: >
            Error: Browser timeout exceeded: 200s
            Error while executing test: ESLint | mirage: mirage/config.js
            Stderr: 
             [0619/093644.920634:ERROR:gpu_process_transport_factory.cc(1007)] Lost UI shared context.

Upgrading ember-cli and testem didn't help. The file in question was lint-free. Our work was paralyzed for days, I was pulling my hair out of helplessness: none of my code changes in the branch were related to the error.

I've found the cause of the issue by chance. It was setting the amount of ember-exam partitions (parallel test runs) to a number larger than the amount of CPU cores available on the CI. I assumed that extra partitions would be queued (I still think they should).

I believe a warning in the readme against an excessive number of partitions can save a lot of frustration.

[lolmaus]

Story of pain and misery: testem/testem#1021 (comment)

[lolmaus]

Issue closed without explanation?