docs: dev: document how to use `dep` `source` entries to test vc with other projects

[grahamwhaley]

Document how to use the dep source entries in Gopkg.toml in other projects to allow testing of vc user branches before PR submission.
In particular I'm thinking of how you would do this in https://github.com/clearcontainers/runtime and https://github.com/kata-containers/runtime.

Fundamentally, a change something like:

diff --git a/Gopkg.toml b/Gopkg.toml
index 1928aa0..24ae553 100644
--- a/Gopkg.toml
+++ b/Gopkg.toml
@@ -24,7 +24,9 @@

 [[constraint]]
   name = "github.com/containers/virtcontainers"
-  revision = "7d4e2f13a04aa73e0d876cdd7f1cf018d15e50bd"
+  source = "https://github.com/grahamwhaley/virtcontainers.git"
+  branch = "20180228_chmod_dirs"

and add that to https://github.com/containers/virtcontainers/blob/master/documentation/Developers.md