Suggest using .git-blame-ignore-revs when adopting ktfmt

[qwwdfsad]

Git has --ignore-revs-file option that allows skipping particular commits during git blame.

By convention, GitHub expects a root .git-blame-ignore-revs file to be used as --ignore-revs-file, as well as GitLab, Black [also] recommends using it.

It would be nice to recommend this approach in the README, both for formatter adopters and for backwards-incompatible changes in the formatter (e.g. #293)

[qwwdfsad]

Unfortunately, IntelliJ doesn't seem to respect this convention (yet), and console git won't do it anyway, so git config blame.ignoreRevsFile .git-blame-ignore-revs is inevitable suggestion