Document clearly in the main readme on how to activate 0_12

A comment along the lines of "use a commit hash from branch xyz to get 0_12 working" would be great.

I updated our repo to 0.12 and it works fine.

I have checked back several times over the last few months but could never figure out what needs to be done. I had to go dig through pages of several issues till I found a comment.

Start in the main README and locate the section covering version 0.12 activation. Add the needed commit-hash guidance from the relevant branch or issue discussion, then verify that a newcomer can follow the README instructions to get 0.12 working.

Written by the indexing model from the issue text.