Updated CONTRIBUTING.md to have up-to-date information about building with Go

CONTRIBUTING.md

@@ -53,35 +53,35 @@ it raises the chances we can quickly merge or address your contributions.
 If you have never worked with Go before, you will have to complete the
 following steps in order to be able to compile and test Packer.
 
-1. Install Go. Make sure the Go version is at least Go 1.4. Packer will not work with anything less than
-   Go 1.4. On a Mac, you can `brew install go` to install Go 1.4.
+1. [Download](https://golang.org/dl) and install Go. Make sure your Go version
+   is at least Go 1.5. Packer _may_ work with older versions of Go but these
+   are not supported.
 
-2. Set and export the `GOPATH` environment variable and update your `PATH`.
-   For example, you can add to your `.bash_profile`.
+2. Set and export the `GOPATH` environment variable and update your `PATH`. For
+   example, you can add to your `.bash_profile`. If you're using Go 1.5 also
+   set `GO15VENDOREXPERIMENT`. This is not necessary for Go 1.6 and up.
 
     ```
     export GOPATH=$HOME/Documents/golang
+    export GO15VENDOREXPERIMENT=1
     export PATH=$PATH:$GOPATH/bin
     ```
 
-3. Install and build `gox` with
+3. Download the Packer source (and its dependencies) by running `go get
+   github.com/mitchellh/packer`. This will download the Packer source to
+   `$GOPATH/src/github.com/mitchellh/packer`.
 
-    ```
-    go get github.com/mitchellh/gox
-    cd $GOPATH/src/github.com/mitchellh/gox
-    go build
-    ```
-
-4. Download the Packer source (and its dependencies) by running
-   `go get github.com/mitchellh/packer`. This will download the Packer
-   source to `$GOPATH/src/github.com/mitchellh/packer`.
+4. When working on packer `cd $GOPATH/src/github.com/mitchellh/packer` so you
+   can run make and easily access other files.
 
-5. Make your changes to the Packer source. You can run `make` from the main
-   source directory to recompile all the binaries. Any compilation errors
-   will be shown when the binaries are rebuilding.
+5. Make your changes to the Packer source. You can run `make` in
+   `$GOPATH/src/github.com/mitchellh/packer` to run tests and build the packer
+   binary. Any compilation errors will be shown when the binaries are
+   rebuilding.
 
-6. Test your changes by running `make test` and then running
-   `$GOPATH/src/github.com/mitchellh/packer/bin/packer` to build a machine.
+6. After running `make` successfully, use
+   `$GOPATH/src/github.com/mitchellh/packer/bin/packer` to build a machine and
+   verify your changes work.
 
 7. If everything works well and the tests pass, run `go fmt` on your code
-   before submitting a pull request.
+   before submitting a pull-request.