chore: add README

Author: vanyae-cqc

tests/data/README.md

@@ -0,0 +1,8 @@
+# Test programs
+
+This directory contains simple quantum programs used for testing.
+
+The compiled HUGR is stored (alongside the guppy python program) with a `.hugr` extension.
+
+Run `just recompile` in this directory (or `just recompile-test-hugrs` on
+the root) to recompile the `.hugr` files.