playground: adding missing README.md

Change-Id: I8c7184d93ba1ab4f3743e9b0a2d13a7b26df3813
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..0bb4aac
--- /dev/null
+++ b/README.md
@@ -0,0 +1 @@
+This repository contains the Vanadium project playground.