Add flutter resource

[kalpeshT101]

What does this PR do?

Add resource(s)

For resources

Description

Add flutter resource

How do we know it's really free?

Artcile

For book lists, is it a book? For course lists, is it a course? etc.

Course

Checklist:

• [ ] Read our contributing guidelines.
• [ ] Search for duplicates.
• [ ] Include author(s) and platform where appropriate.
• [ ] Put lists in alphabetical order, correct spacing.
• [ ] Add needed indications (PDF, access notes, under construction).
• [ ] Used an informative name for this pull request.

[eshellman]

shouldn't this be in courses, or interactive tutorials - there are a lot of videos.