Expand contributing guidelines

[Kalobi]

Add some points that have recently come up to the contributing guidelines. Suggestions welcome.

[SnipUndercover]

Moving this from #modding_dev to here to keep a paper trail.
Markdown has significant trailing whitespace; putting two or more whitespaces at the end of a line inserts a line break (HTML <br/>), as opposed to two blank lines, which create a paragraph break (HTML </p> and <p>).

[SnipUndercover]

Suggested change

	Make sure bug reports have enough information to be reproduced easily!
	Make sure bug reports have enough information to be reproduced easily!\

In my opinion the flow of the document is better with the line breaks preserved, so this applies to all the line diffs where the only change is the removal of the double trailing whitespace.
Fortunately, ending a line with a backslash is another way to achieve a line break that's less prone to breaking due to autoformatters.

This suggestion applies to other diffs where the double trailing whitespace was removed.