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/cloud/README.md b/cloud/README.md
new file mode 100644
index 0000000..eafb512
--- /dev/null
+++ b/cloud/README.md
@@ -0,0 +1,3 @@
+# `//cloud`
+
+`//cloud` contains tooling and libraries for orchestrating Monogon OS clusters.