Update README.md to add slidingWindows(ofCount:) and striding(by:)

[ollieatkinson]

Adds slidingWindows(ofCount:) and striding(by:) to README.md

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[xwu]

👍 One set of nits.

[natecook1000]

Thanks, @ollieatkinson!