Add hint for tagging PRs with link to fixed issue to CONTRIBUTING.md

Adds some documentation:

CONTRIBUTING.md:
- Adds the hint to use "fixes #issue" into the pull request. Questions about this came up in several recent PRs.
- Add the link to issue "Git pull request system" which explains how "fixes #issue" works.
- Spelling correction.

Replaces a tab in the CODE_STYLE example.

Some confusion about the astyle invocation:
CONTRIBUTING.md includes the parameter `--indent-switches`, doc/CODE_STYLE.txt does not include it. The other parameters are the same. What about `--indent-switches`?