Add release note instructions
Describe new release note dev process

For upcoming contributions, every contributor should document the contributed
changes, before merging the PR.
Add release notes for upcoming versions
Remove contributors for current release notes
Rename Changes section to API changes
Add intro to release notes
Add release notes template
Merge pull request #1023 from ahojnnes/release

New release note process
Clarify release procedure doc
Merge pull request #1093 from tonysyu/clarify-release-procedure

Clarify release procedure doc
Fix denoise tests
Merge pull request #1095 from ahojnnes/denoise-tests

Fix denoise tests
Fix isodata-thresholding according to Zachary Pincus
Return to old function name threshold_isodata
Account for case when image only contains one unique value
Fix test cases which are now wrong due to previous binning error
Improve test coverage
Fix return value description