Mention ci-matrix in the main help message

[Pawka]

The gotestsum tool ci-matrix was not mentioned in the main help message. I have found this feature only accidentally :-)

Add the command to the main help message which is printed via gotestsum help:

$ gotestsum help
...
Commands:
    ./gotestsum tool ci-matrix   use previous test runtime to place packages into optimal buckets
    ./gotestsum tool slowest     find or skip the slowest tests
    ./gotestsum help             print this help next

[dnephin]

Thank you for the PR! You're right this new tool is not yet documented in the help text or in the README. I have it on the list of things to do in #287.

I have been hesitant to document this new tool because I'm not sure if the design is correct. #287 has a number of improvements, but I'm also interested in finding a way to avoid the first "matrix building" step. That can add 30s+ to the total runtime, which negates some of the benefit gained by running tests in parallel.

I'm going to hold off on merging this PR until the tool settles a bit. I believe there is at least one large project using the current tool. If you end up using ci-matrix I would love your feedback about it, especially if you are able to use it with buildkite or some other CI system.

[Pawka]

Totally makes sense! I though it simply was forgotten to be added :-) Thank you for explaining.