Merge pull request #719 from sappo/master

Problem: New contributers don't know about best practices and automatic code formatting.
Problem: hashtagged issues aren't automatically hyperlinked in md files

Solution: hard link to issue #680
Problem: some folk seem to be hard of hearing

Solution: extend the Warnings section of CONTRIBUTING. It's sad we need
to do this yet it's clear we lose a lot of time and effort otherwise.
Merge pull request #720 from hintjens/master

Problem: some folk seem to be hard of hearing
Problem: README doesn't refer to contributer guidelines

Solution: Refer to the guidelines in the README at section Hints for contributers