Build instructions for contributors (because it's not obvious)

CONTRIBUTING.md

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+## Prerequisites
+To contribute code changes to this project you will need the following development kits.
+ * Go. [Download and install](https://golang.org/doc/install) the Go programming language
+ * [docker](https://docs.docker.com/engine/installation/)
+
+## Checking out the code
+When cloning watchtower to your development environment you should place your forked repo within the [standard go code structure](https://golang.org/doc/code.html#Organization).
+```bash
+cd $GOPATH/src
+mkdir <yourfork>
+cd <yourfork>
+git clone git@github.com:<yourfork>/watchtower.git
+cd watchtower
+```
+
+## Building and testing
+watchtower is a go application and is built with go commands. The following commands assume that you are at the root level of your repo.
+```bash
+go get ./... # analyzes and retrieves package dependencies
+go build     # compiles and packages an executable binary, watchtower
+go test      # runs tests
+./watchtower # runs the application (outside of a container)
+```
+
+### Building the docker image
+watchtower is packaged and distributed as a docker image. A [golang-builder](https://github.com/CenturyLinkLabs/golang-builder) is used to package the go code and its
+dependencies as a minimally-sized application. The application binary is then layered into to a minimal docker image (see `Dockerfile`), so that the entire image is <10MB.
+See `circle.yml` for further details.The following commands assume that you are at the root level of your repo (i.e. `watchtower/`).
+
+```bash
+docker pull centurylink/golang-builder:latest									# download the builder
+docker run -v $(pwd):/src centurylink/golang-builder:latest						# build the minimal binary
+docker build -t <yourfork>/watchtower:latest .									# build the docker image
+docker run -v /var/run/docker.sock:/var/run/docker.sock centurylink/watchtower	# run the application (inside of a container)
+```
+