Document that 'dep' is required

[candlerb]

Trying to build under ubuntu 16.04 with go 1.10.1, and following the building instructions:

root@builder:~# go get github.com/travisjeffery/jocko
package github.com/travisjeffery/jocko: no Go files in /root/go/src/github.com/travisjeffery/jocko
root@builder:~# cd go/src/github.com/travisjeffery/jocko/
root@builder:~/go/src/github.com/travisjeffery/jocko# make
make: dep: Command not found
Makefile:7: recipe for target 'deps' failed
make: *** [deps] Error 127

Turns out a tool called "dep" needs to be installed; I found installation instructions here and installed a pre-compiled binary.

Note: it gives the following warning:

Warning: the following project(s) have [[constraint]] stanzas in Gopkg.toml:

  ✗  github.com/gorilla/handlers
  ✗  github.com/gorilla/mux
  ✗  github.com/prometheus/client_golang

However, these projects are not direct dependencies of the current project:
they are not imported in any .go files, nor are they in the 'required' list in
Gopkg.toml. Dep only applies [[constraint]] rules to direct dependencies, so
these rules will have no effect.

Either import/require packages from these projects so that they become direct
dependencies, or convert each [[constraint]] to an [[override]] to enforce rules
on these projects, if they happen to be transitive dependencies,

[travisjeffery]

I think I'm gonnna switch to modules as soon as 1.11 is out.