Mention how to configure template_extensions in pyproject.toml

[jaap3]

Through some trial and error I was able to figure out that in order to configure template_extensions in pyproject.toml I have to do the following:

[tool.coverage.django_coverage_plugin]
template_extensions = 'html,htm'

I also discovered that the following doesn't work:

[tool.coverage.django_coverage_plugin]
template_extensions = [
    'html',
    'htm'
]

as an AttributeError: 'list' object has no attribute 'split' is raised by DjangoTemplatePlugin.__init__.