Created flutter.gitignore

[chrisstayte]

Added from Flutter's repository. https://github.com/flutter/flutter/blob/master/.gitignore

Reasons for making this change:

I am adding this file to allow for quick creation of a git ignore in flutter projects via the Github repository creation page.

Links to documentation supporting these rule changes:

Here is a link to the official git ignore that flutter recommends to use. https://github.com/flutter/flutter/blob/master/.gitignore

If this is a new template:

https://github.com/flutter/flutter https://flutter.dev