Look at the address for a given forum. For example the technical issues forum looks like this:
The number at the end is the forum ID.

Now in Google Reader or other reader software enter the following address with the ID at the end. That's the subscription address you need:
I see more than just the first post of each thread. I'm not sure why others said they see just 1 post. In Google Reader you'll notice a button that reads "1 new item" or something like that. Click it and change it to All Items. Now you'll both read and unread. As the feed comes in over time, that list will grow.