Provide makefile to build tool about container-diff

[duggelz]

As a first step, instructions on how to build the tool in the README.md would be good. Not being a Go developer, it seems like I have to name the containing directories in some particular way and set GOPATH to something?

[duggelz]

After getting it working, I have no good idea how to turn it into a Makefile, given that several steps need to happen before even cloning the repo. Maybe with symlinks. For posterity, here's what worked for me on a Ubuntu-like workstation:

# [Go compiler and runtime already installed]
rm -rf ~/code/workspace1
export GOPATH=~/code/workspace1
go get github.com/tools/godep
export PATH=$PATH:~/code/workspace1/bin
cd code/workspace1/
mkdir -p src/github.com/GoogleCloudPlatform
cd src/github.com/GoogleCloudPlatform
git clone [email protected]:GoogleCloudPlatform/container-diff.git
cd container-diff/
godep restore
go get code.cloudfoundry.org/bytefmt  # Presumably can be fixed with godeps somehow
godep go build main.go
./main [arguments]

[r2d4]

I'll take a stab at this