Update _code-of-conduct.md

Some minor additions that could be useful.
Added Mindfullness section. Not sure about how to phrase this part, but thought that the concept would be helpful in the context of this.
Added gender stereotypes in Unacceptable behaviour. Many times we use gender-ed images and figures of speech to explain or underline concepts. It's common but can be felt as offensive.
I wouldn't go into too many details in the text. After all, most of the people in our community are grown ups with more or less the ability to read this text and abstract it for their own behavior. 

I wouldn't want this CoC to look like a text to parent adult people with a wagging finger. See @skade talking about behavior instruction. 

In general: Good thinking, but nothing for this CoC. - My thought.