Tweak Open Collective things (#137)

* Remove redundant Contributors section from README

* Remove Open Collective's ISSUE_TEMPLATE.md

* Tweak Open Collective and badges in README.md

* Fix 'Reference to undefined definition' in README.md

* Fix 'Link to unknown heading(s)' in README.md

* Fix 'Unused definition' in README.md

* Remove CONTRIBUTING.md

* Remove CONTRIBUTING.md and README.md from hallmark ignore option

* Run hallmark