docs: move coding conventions into contributing.md

[leighmcculloch]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

Summary

Move Go coding conventions from the README.md into the CONTRIBUTING.md file which discusses other requirements for code contributed.

Goal and scope

I'm in the process of adding a developing document that includes finer details about how to do things like add dependencies, update them, remove them, as part of the previous work switching to Modules (#1634). I noticed during that work that this section is in the projects readme but is primarily listing more things that someone contributing should be thinking about, and not so much someone who is browsing or importing the repository. It makes more sense for this to live in the contributing document.

Summary of changes

• Copy and paste the code from README.md to CONTRIBUTING.md.

Known limitations & issues

N/A

What shouldn't be reviewed

N/A

[ire-and-curses]

The move is good. The content is lousy but that's a different PR