Add style guide

We should add a style guide so that we can make our code more consistent and easier to read. https://github.com/apple/foundationdb/blob/master/flow/README.md is kind of close to a style guide already, so maybe we can use that.

We should link to it from our PR template: #1559.

Any time we make a style comment on a pull request, we can consider either referencing the style guide or adding to it.

Start by reviewing flow/README.md and the requested PR template change in issue #1559. Identify which existing style guidance is reusable and what project-specific conventions are missing. Done means a proposed style guide is added, linked from the PR template, and its scope is clear enough to reference in future style discussions.

Written by the indexing model from the issue text.