stop claiming that git submodules break bisect

Summary:
If a peru project doesn't put `peru sync` inside its build tools (or if
it has no other build tool), the user still needs to call it in every
step of a bisect, or after a checkout. In this way, it's no better than
`git submodule update`. There maybe be a point about it being more
appropriate for Make to call peru than for it to call git, but I'm not
really sure. Let's just take this part out of the readme.

See: https://www.reddit.com/r/vim/comments/2o6av8/peru_pathogen_a_new_alternative_to_plugin/cmlc4l0

Closes https://github.com/buildinspace/peru/issues/64.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D137