root: move Metropolis README contents to metropolis/README.md, document launch-cluster

Change-Id: I3555ce1d31e83e27239bcd8dde20bdc69deac35c
Reviewed-on: https://review.monogon.dev/c/monogon/+/2765
Tested-by: Jenkins CI
Reviewed-by: Leopold Schabel <leo@monogon.tech>
diff --git a/README.md b/README.md
index 34c51d3..e8b8ec9 100644
--- a/README.md
+++ b/README.md
@@ -19,6 +19,11 @@
 
 ## Monogon OS
 
+The source code lives in `//metropolis` (Metropolis is the codename of Monogon OS).
+
+See the `//metropolis/README.md` for a developer quick start guide, or see
+the [Monogon OS Handbook](https://docs.monogon.dev/metropolis-v0.1/handbook/index.html) for user documentation.
+
 ### Run a single node demo cluster
 
 Build CLI and node image: