feat: add README files for various directories to document structure and usage

configs/README.md

@@ -0,0 +1,13 @@
+# Reproducible Build Profiles
+
+This directory contains committed Make configuration profiles. Each profile
+selects SoC, CPU/IP, PDK, ISA, CSR support, application, linker layout, and
+the supported validation tier.
+
+`ci/` contains pull-request profiles, `nightly/` adds extended open-source
+coverage, and `cluster/` contains configurations requiring site tools or PDKs.
+Start builds from a committed profile rather than setting an unreviewed mix of
+variables on the command line.
+
+For supported profiles and commands, see the root [README](../README.md) and
+[agent contract](../AGENTS.md).