Add sigma

[tosone]

Please check if what you want to add to awesome-go list meets quality standards before sending pull request. Thanks!

Please provide package links to:

• repo link (github.com, gitlab.com, etc): https://github.com/go-sigma/sigma.git
• pkg.go.dev: N/A
• goreportcard.com: https://goreportcard.com/report/github.com/go-sigma/sigma
• coverage service link (codecov, coveralls, etc.): https://app.codecov.io/gh/go-sigma/sigma

Note: that new categories can be added only when there are 3 packages or more.

Make sure that you've checked the boxes below that apply before you submit PR.
Not every repository (project) will require every option, but most projects should. Check the Contribution Guidelines for details.

• The package has been added to the list in alphabetical order.
• The package has an appropriate description with correct grammar.
• As far as I know, the package has not been listed here before.
• The repo documentation has a pkg.go.dev link.
• The repo documentation has a coverage service link.
• The repo documentation has a goreportcard link.
• The repo has a version-numbered release and a go.mod file.
• I have read the Contribution Guidelines, Maintainers Note and Quality Standards.
• The repo has a continuous integration process that automatically runs tests that must pass before new pull requests are merged.
• Continuous integration is used to attempt to catch issues prior to releasing this package to end-users.

Thanks for your PR, you're awesome! 👍

[github-actions]

Thank you for contributing with awesome-go, we will revise your contribution as soon as possible.

Make sure you add the links in the body of the pull request that are requested in the contribution guide:

• godoc.org or pkg.go.dev
• goreportcard.com
• coverage

your project is about to be approved, it's under revision, it may take a few days