fix: go module requirements for semantic versioning

This commit also adds the compatibility with dep (Go dependency
management tool) via symlink creation (same method applied in
kubernetes-csi/csi-test)

TODO: fix hack/update-generated-code.sh

Fixes issue #239.

Signed-off-by: Alex Szakaly <alex.szakaly@gmail.com>

v2/README.md

@@ -0,0 +1,18 @@
+This directory mirrors the source code via symlinks.
+This makes it possible to vendor v2.x releases of
+external-snapshotter 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 external-snapshotter in their Gopk.toml, like this:
+
+```toml
+[prune]
+  go-tests = true
+  unused-packages = true
+
+  [[prune.project]]
+    name = "github.com/kubernetes-csi/external-snapshotter"
+    unused-packages = false
+```