Update Wagtail installation instructions in README.md

[nimasmi]

This adds mention of importing the template tag library, and adding kontrasto to INSTALLED_APPS.

Note also that the current version in the repository is not the latest on PyPI: the wgac_3_contrast_light_or_dark template tag is not available in 0.1.0 on PyPI. Maybe a version tag and a release in Git would help to iron out that confusion.

[netlify[bot]]

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