Improve formatting of contributing.md

- Added spaces at the end of many lines to prevent the words from
  sticking to the words at the next line.
- Removed superfluous <br/>s
- Fixed typo (impleemented -> implemented)
- Turned the two links at the end in a clickable link.
- Fixed code formatting errors (backticks/indention).
Merge pull request #165 from Rob--W/docs/contributing-typos

Improve formatting of contributing.md