Adding some README badges for downloads, license and stable version.
Fixing broken docs on setting allowed extensions on a FileField

Refers to an invalid variable, and is overriding the entire Upload,
which isn't necessary.
Merge pull request #3478 from halkyon/readme_badges

Adding some README badges for downloads, license and stable version.
Merge pull request #3480 from halkyon/allowed_exts_doc_change

Fixing broken docs on setting allowed extensions on a FileField
Refactored CSV export functionality to sit in it's own class.

Relates to #2802 #2431#3408
type hint boolean instead of bool
PHPDoc and instantiation updates
Merge branch 'features/csv-export-refactor' into 3.1-jtch