Add decimal to Financial

[eapenkin]

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/govalues/decimal
• pkg.go.dev: https://pkg.go.dev/github.com/govalues/decimal
• goreportcard.com: https://goreportcard.com/report/github.com/govalues/decimal
• coverage service link (codecov, coveralls, etc.): https://app.codecov.io/gh/govalues/decimal

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.
• The authors of the project do not commit directly to the repo, but rather use pull requests that run the continuous-integration process.

Thanks for your PR, you're awesome! 👍

[panjf2000]

Is there anything else you can work on this gocyclo issue?

[panjf2000]

gocyclo issue needs to be fixed.

[eapenkin]

@panjf2000, I updated the code.