Document --assume-in-merge in README

Committed via https://github.com/asottile/all-repos

Re: https://github.com/pre-commit/pre-commit-hooks/blob/40cc31b02ceb72045509576e51d9c82503751422/pre_commit_hooks/file_contents_sorter.py#L54-L65

Closes https://github.com/pre-commit/pre-commit-hooks/issues/543

New hook 'destroyed-symlinks' to detect symlinks which are changed to regular files with a content of a path which that symlink was pointing to; move zsplit to util

The --enforce-all option when provided ensures that all files passed
on the command line are checked against the size limit.  Default
behaviour remains unchanged.

Include instructions on how to use files/types/excludes/exclude_types
with the no-commit-to-branch hook, along with the caveat of it allowing
empty commits