README: add a link to the base-buildkite-docs repository (#41956)

DilumAluthge · KristofferC · commit 1b13f82026d5 · 2021-09-03T12:20:14.000+02:00
(cherry picked from commit 310bf16)
diff --git a/.buildkite/README.md b/.buildkite/README.md
@@ -3,3 +3,5 @@
 This directory contains the Buildkite configuration files for Base Julia CI.
 
 The rootfs image definitions are located in the [rootfs-images](https://github.com/JuliaCI/rootfs-images) repository.
+
+The documentation for the Base Julia CI setup is located in the [base-buildkite-docs](https://github.com/JuliaCI/base-buildkite-docs) repository.
