Add README to legacy_features directory

As suggested in #408, this adds a signpost until do the work in #408 to
fix the actual broken window.

legacy_features/README.md

@@ -0,0 +1,14 @@
+# Please do not add to the tests in this folder
+
+This folder contains the acceptance tests that were written for Cucumber
+before [Aruba](https://github.com/cucumber/aruba) existed.
+
+These tests are currently run as part of the build, but should not be added to.
+
+New acceptance tests should be added in the `features` directory. If you find a test
+in here that you want to modify, please do the work to migrate it into the `features`
+directory.
+
+There is a ticket [#408](https://github.com/cucumber/cucumber/issues/408) to track 
+the effort to migrate all these tests into the `features` directory. We aim to have this
+completed for the release of Cucumber 2.0