update CONTRIBUTING.md

[skip ci]

CONTRIBUTING.md

@@ -77,4 +77,14 @@ In separate terminals:
 If you want to export the current data in the Mastodon instance as canned data, 
 so that it can be loaded later, run:
 
-    npm run backup-mastodon-data
+    npm run backup-mastodon-data
+
+## Writing tests
+
+Tests use [TestCafé](https://devexpress.github.io/testcafe/). The tests have a naming convention:
+
+* `0xx-test-name.js`: tests that don't modify the Mastodon database (post, delete, follow, etc.)
+* `1xx-test-name.js`: tests that do modify the Mastodon database
+
+In principle the `0-` tests don't have to worry about
+clobbering each other, whereas the `1-` ones do.