And use single quotes consistenly.

The current numbers are misleading and coverage tracking doesn't work in
most of the places since the current test suite is mostly based on
spawning subprocesses, and coverage doesn't get properly tracked there.

Let's revisit this in the future.

Per @hmistry's suggestions, I've moved the development instructions outside of the bulleted list and made it its own sentence. And I've shortened the copy under the "Contributing to Bundler" header.

Hey team,

As part of an ongoing effort to revamp the docs, I've been working with @indirect to rewrite the contributor guidelines README. We wanted to streamline the way the information was laid out, and make it more explicit to potential contributors how they can contribute and what next steps they needed to take. We condensed everything down to three sections (a general guidelines section, contributor section, and a new, supporting Bundler section). Everything links out to existing documentation.