ci: Add hint about downloadable logs to README

I (Andres) chose to backpatch this to 15, as it seems better to keep the
README the same.

Author: James Coleman <jtc331@gmail.com>
Discussion: https://postgr.es/m/CAAaqYe_7BXDjpk0Ks_eqf1r6LZpC_rfB7kjhb_T3+eC4t6yiGQ@mail.gmail.com
Backpatch: 15-, where CI came in

src/tools/ci/README

@@ -37,6 +37,10 @@ repository will automatically trigger CI builds. These are visible from the
 commit history / PRs, and can also be viewed in the cirrus-ci UI at
 https://cirrus-ci.com/github/<username>/<reponame>/
 
+Hint: all build log files are uploaded to cirrus-ci and can be downloaded
+from the "Artifacts" section fron the cirrus-ci UI after clicking into a
+specific task on a build's summary page.
+
 
 Images used for CI
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