Document missing development step
Document missing development step

Since the project now depends on github.com/vektra/errors, document this in order to avoid the following error:

```
go build ./cmd/puma-dev
cmd/puma-dev/command.go:11:2: cannot find package "github.com/vektra/errors" in any of:
	/usr/local/Cellar/go/1.7.3/libexec/src/github.com/vektra/errors (from $GOROOT)
	/usr/local/var/golang/src/github.com/vektra/errors (from $GOPATH)
make: *** [all] Error 1
```
attilagyorffy/puma-dev,attilagyorffy/puma-dev,puma/puma-dev,puma/puma-dev,attilagyorffy/puma-dev,puma/puma-dev,attilagyorffy/puma-dev,puma/puma-dev