document reason for 0.21.2 having been yanked from crates.io

I just noticed that 0.21.2 was recently (i.e. 1-2 weeks ago) yanked from crates.io, but couldn't find any issue or pull request that would explain why that happened (i.e. a search for "yank" yields no relevant issues or pull requests). We have already updated the Fedora Linux package for rust-ini to 0.21.2 a while ago, so it would be good to know for us if we need to roll back to 0.21.1 or backport some kind of bugfix.