Add documentation about not making changes directly to master branch or performing squash merges or rebase merges

[notmessenger]

Per the conversation in #142 making changes directly to the master branch of a repo without the use of a PR or the merging of a PR as either a squash merge or a rebase merge will cause a new version to released with the previous PR's scope and CHANGELOG.md contents to be used. This is not desirable, but per #142 (comment) it has not yet been determined how this can be prevented programmatically so in the meantime it should be noted in the README.md that this behavior should be avoided.

[job13er]

FYI: I figured out a way to support squash and rebase merging in GitHub. Not sure if the process will work for bitbucket or any other vcs, but the GitHub API gives a merge_commit_sha in the pull request description that you can match against the commit you're building. Here's how implemented it in bumpr (my pr-bumper implementation): https://github.com/jobsquad/bumpr/blob/v2.1.0/src/vcs/github.js#L72-L96