Looks Good To Me
I added a small LGTM button to the top-level discussion's reply field, similar to the Done button that shows up for the PR author.
This can be a convenient way to indicate that you think the PR is good to merge, but you can also configure your repository to require such an approval to complete a review. This will be reflect in Reviewable's status checks on the PR and, in turn, you can configure GitHub to require successful checks before permitting a merge.
This has actually been possible for a few months now but I figured I'd publicize it a bit more! :-)