Add a basic readme.md to the regression folder.

In order to explain what tests marked `winbug` are
about.

Author: NlightNFotis

regression/readme.md

@@ -0,0 +1,12 @@
+# CProver regression tests
+
+This folder contains the CProver regression test-suite.
+
+## Notes
+
+* Tests marked as `winbug` are currently known to be failing
+  on Windows, but passing on other platforms. The reason for
+  this is not known, and it's currently being investigated.
+  This was discovered during work done to port CI from travis
+  and codebuild to github actions. Worth noting that those tests
+  were not being run on Windows before.