Update README.md install section.

This commit adds an additional step to the README.md install section to
run the go version command and check the version so people that are
installing it for the first time and ensure they are running a high
enough version and have GOROOT and GOPATH set correctly.

README.md

@@ -48,10 +48,22 @@ https://github.com/btcsuite/btcd/releases
 - Install Go according to the installation instructions here:
   http://golang.org/doc/install
 
+- Ensure Go was installed properly and is a supported version:
+
+```bash
+$ go version
+$ go env GOROOT
+$ go env GOPATH
+```
+
+NOTE: The `GOROOT` and `GOPATH` above must not be the same path.  It is
+recommended that the `GOPATH` is set to a directory in your home directory such
+as `~/goprojects` to avoid write permission issues.
+
 - Run the following command to obtain btcd, all dependencies, and install it:
 
 ```bash
-$ go get github.com/btcsuite/btcd/...
+$ go get -u github.com/btcsuite/btcd/...
 ```
 
 - btcd (and utilities) will now be installed in either ```$GOROOT/bin``` or