Fix readme to say use `go get` instead of `go install` to get (and install) go-extpoints

[petemoore]

Note, go get will fetch and install, whereas go install will just install, but if it is not already in the go workspace, will report an error, e.g.

$ go install github.com/progrium/go-extpoints
can't load package: package github.com/progrium/go-extpoints: cannot find package "github.com/progrium/go-extpoints" in any of:
    /usr/local/Cellar/go/1.5.1/libexec/src/github.com/progrium/go-extpoints (from $GOROOT)
    /Users/pmoore/go/src/github.com/progrium/go-extpoints (from $GOPATH)

With go get both fetching and installing are achieved:

$ go get github.com/progrium/go-extpoints
$ which go-extpoints 
/Users/pmoore/go/bin/go-extpoints

[petemoore]

@progrium Just a small change here. Thanks!