Add config file to enable Buildomat CI for illumos

.github/buildomat/README.md

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+# Buildomat illumos CI
+
+This directory contains CI configurations for the [illumos] operating system.
+Tokio's illumos CI jobs are run using [Buildomat], a CI system developed by
+Oxide Computer, which supports illumos. See [the Buildomat README] for more
+details.
+
+## illumos-Specific CI Failures
+
+If your pull request's CI build fails on illumos, and you aren't able to easily
+reproduce the failure on other operating systems, don't worry! The
+[tokio-rs/illumos] team is responsible for maintaining Tokio's illumos support,
+and can be called on to assist contributors with illumos-specific issues. Please
+feel free to tag @tokio-rs/illumos to ask for help resolving build failures on
+illumos 
+
+[illumos]: https://www.illumos.org/
+[Buildomat]: https://github.com/oxidecomputer/buildomat
+[the Buildomat README]: https://github.com/oxidecomputer/buildomat
+[tokio-rs/illumos]: https://github.com/orgs/tokio-rs/teams/illumos

.github/buildomat/config.toml

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+# Repository-level Buildomat configuration.
+# See: https://github.com/oxidecomputer/buildomat#per-repository-configuration
+
+# Enable buildomat. This one should be self-explanatory.
+enable = true
+# Allow CI runs for PRs from users outside the `tokio-rs` organization. Our
+# buildomat jobs don't touch any secrets/keys, so this should be fine.
+org_only = false