doc: add collaborator guide

llnode has been unofficially following the Node.js Collaborator Guide
wrt Approving and Landing PRs policy. This commit adds
COLLABORATOR_GUIDE.md with a link to Node.js Collaborator Guide, so
we'll start to follow these guidelines officially.

Fixes: #242

PR-URL: #275
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

Author: mmarchini

COLLABORATOR_GUIDE.md

@@ -0,0 +1,4 @@
+# llnode Collaborator Guide
+
+llnode follows the
+[Node.js Collaborator Guide](https://github.com/nodejs/node/blob/master/COLLABORATOR_GUIDE.md).