Add gobl to financial libraries list

[samlown]

Added gobl to the list of financial libraries.

We want to ensure high quality of the packages. Make sure that you've checked the boxes below before sending a pull request.

• I have read the Contribution Guidelines
• I have read the Maintainers Note
• I have read the Quality Standards

Not every repository (project) will require every option, but most projects should. Check the Contribution Guidelines for details.

• 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.
• 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.

Please provide some links to your package to ease the review

• forge link (github.com, gitlab.com, etc): https://github.com/invopop/gobl
• pkg.go.dev: https://pkg.go.dev/github.com/invopop/gobl
• goreportcard.com: https://goreportcard.com/report/github.com/invopop/gobl
• coverage service link (codecov, coveralls, etc.): https://codecov.io/gh/invopop/gobl

Pull Request content

• 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.

Category quality

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Packages added a long time ago might not meet the current guidelines anymore. It would be very helpful if you could check 3-5 packages above and below your submission to ensure that they also still meet the Quality Standards.

Please delete one of the following lines:

• The packages around my addition still meet the Quality Standards.
• I removed the following packages around my addition: (please give a short reason for each removal)

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[github-actions]

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• repo link
• pkg.go.dev
• goreportcard.com
• coverage

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