Add readme for v3 dir

v3/README.md

@@ -0,0 +1,18 @@
+This directory mirrors the source code via symlinks.
+This makes it possible to vendor v3.x releases of
+csi-test with `dep` versions that do not support
+semantic imports. Support for that is currently
+[pending in dep](https://github.com/golang/dep/pull/1963).
+
+If users of dep have enabled pruning, they must disable if
+for csi-test in their Gopk.toml, like this:
+
+```toml
+[prune]
+  go-tests = true
+  unused-packages = true
+
+  [[prune.project]]
+    name = "github.com/kubernetes-csi/csi-test"
+    unused-packages = false
+```