[Bug] Invalid hyperlinks in README

[YogeshSinghChouhan]

Scenario

Invalid hyperlinks in README.md, navigating to mentioned links results in page not found (404) error.

Steps to Reproduce

1. Go to https://github.com/SortableJS/Sortable#options
2. Click on docs/sortable-options
3. Go to https://github.com/SortableJS/Sortable#extra-plugins-included-in-complete-versions
4. Click on MultiDrag / Swap
5. Go to https://github.com/SortableJS/Sortable#default-plugins-included-in-default-versions
6. Click on Autoscroll / OnSpill

Describe the bug

Above mentioned links are invalid

Expected behavior

Links should work.

Additional Context

Invalid Link :-
https://github.com/SortableJS/Sortable/tree/master/docs/sortable-options
Replace it with :-
https://github.com/SortableJS/Sortable/blob/master/docs/sortable-options.md

Invalid Link :-
https://github.com/SortableJS/Sortable/tree/master/plugins/MultiDrag
Replace it with :-
https://github.com/SortableJS/Sortable/tree/master/packages/plugins/multi-drag

Invalid Link :-
https://github.com/SortableJS/Sortable/tree/master/plugins/Swap
Replace it with :-
https://github.com/SortableJS/Sortable/tree/master/packages/plugins/swap

Invalid Link :-
https://github.com/SortableJS/Sortable/tree/master/plugins/AutoScroll
Replace it with :-
https://github.com/SortableJS/Sortable/tree/master/packages/plugins/auto-scroll

Invalid Link :-
https://github.com/SortableJS/Sortable/tree/master/plugins/OnSpill
Replace it with :-
https://github.com/SortableJS/Sortable/tree/master/packages/plugins/on-spill

[waynevanson]

Since you've got all the information, you should submit it as a PR.

If you don't, I'll happily do it.

[YogeshSinghChouhan]

Created PR @waynevanson