Mention v2 first

[coilysiren]

Surfaced via @skillful-alex here #925 (comment), people reading the documentation are under the impression that they should be reading the v1 documentation, when they should instead be using v2.

This PR changes the readme to mention the v2 documentation first, and adds a "if you don't know, you're probably using v2" mention.

[codecov]

Codecov Report

Merging #927 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #927   +/-   ##
=======================================
  Coverage   72.89%   72.89%           
=======================================
  Files          32       32           
  Lines        2439     2439           
=======================================
  Hits         1778     1778           
  Misses        550      550           
  Partials      111      111

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f6f1ac6...7739b8c. Read the comment docs.

[xordspar0]

Don't know what version you're on? You're probably using the version from the master branch, which is currently v2.

This is the most helpful part to me. I've known about the (previously dormant) v2 branch for a while, but it's good to know that this is the master branch.

[saschagrunert]

LGTM

[AudriusButkevicius]

I guess it's not really a beta, but rather early days, as beta implies that people should use v1 for something stable, which I don't think is what we want.

[coilysiren]

I'll think on the wording here for a bit, and stand up a PR for changing this line 👍