Create Flutter-cheatsheet.md

[Navaneethp007]

What does this PR do?

Add content(s)

Description

I have added a cheatsheet for flutter

Screenshots or Links

If applicable, add screenshots or a modified UI link to help understand your contribution.

Additional context

Add any other context about your PR.

Checklist:

• Read our contributing guidelines.
• Search for duplicates.
• Used an informative name for this pull request.

Follow-up

• Check the status of GitHub Actions and resolve any reported warnings!

[github-actions]

It's great having you contribute to this project

Welcome to the community 🤓

Thanks for joining our community - we help and encourage each other to contribute to open source little and often 🤓 . Any questions let us know.

[Navaneethp007]

When will the reviewing process start

[crescentpartha]

Thank you for your participation.
Some requirements you need to meet to get a successful PR.

1. Rename your cheat sheet file from Flutter-cheatsheet.md to flutter-cheatsheet.md
2. Add your cheatsheet file in README.md file.
3. Follow the cheatsheet structure like git-cheatsheet.md when you introduce a new cheatsheet.
4. Try to give a tabular format, if it possible.
5. Give a table of contents section which contains all the navigation links section-wise.
6. Keep your cheatsheet simple, readable, and understandable so that developers could review it quickly and find their desired commands, keyboard shortcut, or anything else.
7. Never use unnecessary commits.
8. Finally, make sure you read the contributing guidelines before your final submission.