Improve CONTRIBUTING.md

Improve CONTRIBUTING.md

- Remove "Use English". That's understood since the bug template is in English.
- Put the instruction about bugs at the top.
- Make the paragraph about the changelog a bullet point. It makes the
  instructions look shorter. Since the "Use English" bullet point was removed,
  the list does not look too long either.
- Move the "Contributing code?" link off to the right to get it out of the way.
- Use fewer words in a few places. The less to read the better.

Also: Link to CONTRIBUTING.md instead of directly to the issue tracker in the
readme, to increase the chance of people actually reading it. Therefore a "Go to
the issue tracker" link was added in CONTRIBUTING.md (it goes to the issues
list, not a new issue, to increase the chance of people looking for duplicates).