Add advice on semantic line breaks to CONTRIBUTING.md

## How can Bevy's documentation be improved?

Breaking your lines after each sentence / clause is useful for reviewers and should be the default style for both doc comments and markdown.

To be specific:

1. It makes the git history more reflective of the actual changes.
2. It's much easier to insert (and read) suggestions on Github.
3. It's easier to identify violations of markdownlint on smaller lines.

I find text broken in this way to be much easier to read and understand, but others feel the opposite way :p

We should add a link to the [semantic line breaks](https://sembr.org/) document to the CONTRIBUTING.md section on docs.