Document available packages in TESTS.md

docs/TESTS.md

@@ -84,12 +84,24 @@ library:
  - bar
 ```
 
-Then, when submitting your solution, remember to submit `package.yaml` as
-well (otherwise the solution will fail). From the project's top-level
-directory run:
-
-```bash
-exercism submit package.yaml src/ModuleName.hs
+While working locally, you can use any packages you like.
+The online test runner, however, has access to only a limited set of packages.
+The available packages are
+
+- the ones that come pre-installed with GHC (most notably `base`, `containers`, `array`, `time`, `mtl`, `deepseq`, `exceptions`, `bytestring`), plus
+- those pre-built for the test runner, listed at [haskell-test-runner/pre-compiled/package.yaml](https://github.com/exercism/haskell-test-runner/blob/main/pre-compiled/package.yaml).
+
+If you submit a solution that uses a package that is not available, then the test runner will run into its lack of internet access and produce an error that looks like this:
+
+```text
+Cabal file info not found for unavailable-package-1.0@..., updating
+Selected mirror https://hackage.haskell.org/
+Downloading root
+HttpExceptionRequest Request {
+ host = "hackage.haskell.org"
+ ...
+}
+ (ConnectionFailure ...)
 ```
 
 ### Running *GHCi*