Add vndr to go-compile and add vendoring docs

This makes it easier to safely udpate the vendor directory

Signed-off-by: Dave Tucker <dt@docker.com>

docs/vendoring.md

@@ -0,0 +1,57 @@
+vendoring
+=========
+
+The Go code in this repo depends on a number of Go libraries.
+Theses are vendored in to the `vendor` directory using [`vndr`](https://github.com/lk4d4/vndr)
+The `vendor.conf` file contains a list of the repositories and the git SHA or branch name that should be vendored
+
+## Updating dependencies
+
+Update `vendor.conf` with the dependency that you would like to add.
+Details of usage of the `vndr` tool and the format of `vendor.conf` can be found [here](https://github.com/LK4D4/vndr/blob/master/README.md)
+
+Once done, you must run the `vndr` tool to add the necessary files to the `vendor` directory.
+The easiest way to do this is in a container.
+
+## Updating in a container
+
+To update all dependencies:
+
+```
+docker run -it --rm \
+-v $(PWD):/go/src/github.com/docker/moby \
+-w /go/src/github.com/docker/moby \
+--entrypoint /go/bin/vndr \
+mobylinux/go-compile:90607983001c2789911afabf420394d51f78ced8
+```
+
+To update a single dependency:
+
+```
+docker run -it --rm \
+-v $(PWD):/go/src/github.com/docker/moby \
+-w /go/src/github.com/docker/moby \
+--entrypoint /go/bin/vndr \
+mobylinux/go-compile:90607983001c2789911afabf420394d51f78ced8 \
+github.com/docker/docker
+```
+
+## Updating locally
+
+First you must install `vndr` and ensure that `$GOPATH/bin` is on your `$PATH`
+
+```
+go get -u github.com/LK4D4/vndr
+```
+
+To update all dependencies:
+
+```
+vndr
+```
+
+To update a single dependency:
+
+```
+vndr github.com/docker/docker
+```