I believe you need to whitelist https://docs.google.com in your allowed iframes admin setting, and then place your link in the post something like this:
<iframe src="https://docs.google.com/document/d/1xM9RtF1wp1m50rLcrZ0RBV2OV1JxweyRhJQnAYAaDf8/edit?embedded=true" width="300" height="300" frameborder="0" marginheight="0" marginwidth="0">
</iframe>
Somebody may have more experience though, and can add some extra detail. 