doc: Update test/README.md

Author: rvl

test/README.md

@@ -1,6 +1,17 @@
 ### Haskell infrastructure test cases
 
-To execute the test cases, build from this directory:
+To build the test cases, run from this directory:
 
     nix-build --no-out-link default.nix
 
+To run all tests (includes impure tests), use the script:
+
+    ./tests.sh
+
+
+#### Generated code
+
+If you change the test Cabal files or need to regenerate the code with
+nix-tools, then see `regen.nix`. Run it like this:
+
+    $(nix-build --no-out-link regen.nix)