Clarify maintenance status in readme

[SethTisue]

No description provided.

[SethTisue]

gah, I see this repo is still on travis-ci.org and needs to be moved to travis-ci.com (or GitHub Actions swapped in instead)

@jason-yeo-paidy are you by any chance a Github Actions enthusiast who would like to send a PR going that route? if not, I'll flip the Travis-CI org->com switch

(same question to anyone watching the repo, actually)

although hmm.... hey @eed3si9n is the sbt org hooked up to a paid Travis-CI account? because if not, that actually forces going the GitHub Actions route

[SethTisue]

I migrated the repo to travis-ci.com, but it appears to me at https://travis-ci.com/organizations/sbt/plan/usage that the sbt org does not have a paid plan, and thus this only suffices as a temporary measure.

I've opened #184 and #185 asking for volunteers to add GitHub Actions instead.