docs : add `touch WORKSPACE`
It seems that we still need to touch a file named `WORKSPACE`
```
ERROR: The 'build' command is only supported from within a workspace.
```
Closes
@@ -426,6 +426,7 @@ Package path elements may be specified in three formats:
<pre>
% mkdir -p foo/bazel
% cd foo/bazel
+ % touch WORKSPACE
% bazel build --package_path /some/other/path //foo
</pre>
<h3 id='target-patterns'>Specifying targets to build</h3>
