Add Nullstack from Christian Mortaro

[GuiDevloper]

Contribution Checklist:

• [x] The project was created by the developers born in Brazil
• [x] The project can't be just a personal, experimental or sample project
• [x] The project has more than 100 stars on Github
• [x] Place them in alphabetical order
• [x] One repository per PR/contribution - avoid conflicts and simplify review
• [x] Check if your suggestion follows our pattern (title, description, badge), you need to add double blank space after ending line of each line

Very thanks for creating this initiative, there's too much amazing brazilian projects! And feel free to know about us, the Nullstack initiative! :)