Plugin/workflow when user adds a Document to Opportunity

Hi,

I've installed Out of the box Sharepoint in CRM 13 and I would like to know which will be the best approach to detect when a Document is uploaded to an opportunity.

The approach that I tried was to add a workflow to SharePoint document Entity in the create event, this to trigger the workflow whenever the user add's a record(document) to an opportunity, but as long as I can see I'm not able to add workflows to that entity. There are other related entities but I don't know if they could help on this task, they are the following ones:

1. SharePoint Document(the one mentioned above)

2. Document Location

3. SharePoint site

For you guys, which would be the best way to do what I want?

Thanks in advance!