add note about windows

DEVELOPERS.md

@@ -5,6 +5,13 @@ contribute to Specimin. We welcome improvements, bug fixes, new test
 cases, and any other contributions that you'd like to make. To suggest
 a change to Specimin, please open a [GitHub pull request](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request).
 
+### Development Environment
+
+Specimin's testing infrasture relies on the presence of a `bash` interpreter
+and the availability of the standard Unix `diff` command, so the Specimin tests
+don't natively run on Windows. To work on Specimin, you should either do your work
+on a Unix machine or install [WSL](https://en.wikipedia.org/wiki/Windows_Subsystem_for_Linux).
+
 ### Testing infrastructure
 
 Even Specimin's "unit tests" are actually full system tests: each