Previously, the link wasn't working from Github's rendered markdown.
Another alternative is to add `malto:` to the link, which would make it work. However, I thought that since the email is the most important piece of information in the entire file, we should just write it out.
I've set up a jj-security@googlegroups.com list. The template comes
from Google's internal web.
I have no experience with GitHub's Security Advisory database, but it
seems like a good practice, so let's use it.