Document more differences to compiletest-rs

Author: oli-obk

ui_test/README.md

@@ -48,3 +48,6 @@ their command specifies, or the test will fail without even being run.
 
 * `ignore-*` and `only-*` opereate solely on the triple, instead of supporting things like `macos`
 * only `//~` comments can be individualized per revision
+* no "silent mode" (one `.` per test, instead of printing names)
+* only supports `ui` tests
+* tests are run in named order, so you can prefix slow tests with `0` in order to make them get run first