update build to fix publish

update how to publish, doc index

Merge pull request #25 from theolundqvist/master

add 'updated' method to Image

bump version

Merge pull request #21 from fritjof-b/master

Added simple CI using GH actions and build status to readme, fix #19

add link to sbt-pgp in publish instructions

Merge pull request #5 from jos-rosenqvist-3812/getpixel-require

Improve getPixel error handling

update root doc page with new package contents and version info

add publishing instructions for maintainers

update README, add usage instructions