Add non-Go-user friendly build instructions (#1071)

Go is not  super helpful if you checkout the code to a random path, then try and follow the build instructions in the README. Add some instructions that help users who don't deal with Go on a day-to-day basis to get it right.

Fixes #1070. Thanks to @johnbelamaric for reminding me about Go's deficiencies.

README.md

@@ -65,16 +65,21 @@ Caddyfile when forked).
 ## Compilation
 
 CoreDNS (as a servertype plugin for Caddy) has a dependency on Caddy, but this is not different than
-any other Go dependency. If you have the source of CoreDNS, get all dependencies:
+any other Go dependency. If you have the source of CoreDNS checked out in the appropriate place in
+your `GOPATH`, get all dependencies:
 
     go get ./...
 
+(You can do the checkout and dependency resolution as a single step with: `go get github.com/coredns/coredns`.)
+
 And then `go build` as you would normally do:
 
     go build
 
 This should yield a `coredns` binary.
 
+Note that CoreDNS currently requires at least Go 1.9 to build.
+
 ## Examples
 
 When starting CoreDNS without any configuration, it loads the `whoami` middleware and starts