Improve Docs on Header anchors in Markdown

Problem to solve

Explain better how to link to a header inside an issue.

Further details

Example: Header 1 => that would generate the link header-1 according to the docs, but the docs don't explain that I have to use (My Link to header 1)[#header-1].

I have lost 20-30 minutes until I found this on Google which explains it: https://gist.github.com/asabaylus/3071099

Proposal

Add an according example to the docs.