Introduce GitHub Actions (closes #109)

[atusy]

Summary

• Add .github/workflows/unittest.yml
• Add badge in README.md

Related Issues/Wiki/Resources

• 110

[atusy]

Seems like GitHub Actions is not running in this PR maybe because of account ownership...? Maybe it will work after merge.

You can see how it works under https://github.com/atusy/spm-image/pull/1

[sutkss]

LGTM