[WIP] Restructure and expand CONTRIBUTING

### Changelog:
* Restructure the `Fixing an issue` section:
  * Juxtapose sections about original and planned features.
  * Write a detailed guide on submitting bugs.
* Include code documentation and PR documentation in the requirements section.
* Install consistent markdown.
* Occasionally improve the prose.
* Limit the line length to 79 chars, save for long links.
We are bug-free, aren't we? :P