Description of the Change

Added documentation to filter and updated README.
(Full props to Peter. I only raised PR as he had issues pushing his branch.)

Closes #168

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Changelog Entry

Added: documentation to filter

Credits

Props @peterwilsoncc