Go layer requires gocode

Make this clear in the documentation. Otherwise, the user will see the following periodically in the Emacs status line:

```
 (file-error Searching for program no such file or directory gocode)
```

contrib/lang/go/README.md

@@ -8,8 +8,11 @@
 - [go contribution layer for Spacemacs](#go-contribution-layer-for-spacemacs)
     - [Description](#description)
     - [Install](#install)
+        - [Pre-requisites](#pre-requisites)
         - [Layer](#layer)
     - [Working with Go](#working-with-go)
+        - [Go commands (start with `m`):](#go-commands-start-with-m)
+        - [Go Oracle](#go-oracle)
 
 <!-- markdown-toc end -->
 
@@ -24,6 +27,16 @@ Features:
 
 ## Install
 
+### Pre-requisites
+
+You will need gocode:
+
+```
+go get -u github.com/nsf/gocode
+```
+
+Make sure that `gocode` executable is in your PATH.
+
 ### Layer
 
 To use this contribution add it to your `~/.spacemacs`