Skip builds on specific CI systems

Most CI systems will not build if a git commit -m message includes a string like [skip ci]. For example:

git commit -m "update install docs [skip ci]"

Some CI systems have additional custom commit keywords that allow skipping only that CI:

• GitHub Actions: [skip actions]
• Azure Pipelines: [skip azp]
• Travis-CI: [skip travis]
• AppVeyor: [skip appveyor]
• CircleCI: [skip circle]
• GitLab: git push -o ci.skip git push option
• Bitbucket [skip ci]