New post: announce the upcoming default Leiningen version change

Not sure if this is the blog repo these days. I've been promising @joshk to write an announcement of the default Leiningen version change to 2.x for months. The post is finally here. I'll submit PRs for the actual cookbook, build and docs changes next week.

cc @roidrage
@joshk Action is required on the users' part, so , we should have a clearer timeline than "in the next few weeks".