Document top-level directories

Change-Id: I52889e4823cbeb841c7dc9da12b5fe4c6a79d663
Reviewed-on: https://review.monogon.dev/c/monogon/+/3310
Tested-by: Jenkins CI
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
diff --git a/third_party/README.md b/third_party/README.md
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+# `//third_party`
+
+Anything related to importing third party code and making it work with our monorepo lives here:
+
+- Vendored code.
+- Bazel rules for third party code.
+- Patches to thid party code.
+- Lock files and similar package manager definitions (whenever possible - things like `go.mod  have to live at the top level, and that's OK too).
+
+First-party code or build rules based *on* a third-party component should not live here.