Document boolean attributes

docs/source/basics.rst

···
 >>> print(div(attr='a value with "quotes" and a <tag>'))
 <div attr="a value with &quot;quotes&quot; and a &lt;tag&gt;"></div>
 
+Boolean attributes
+^^^^^^^^^^^^^^^^^^
+
+A boolean HTML attribute is one that does not have a value. For example, to
+mark an input field as required, you can write ``<input required>``. There is
+also an alternative syntax, which minihtml uses: ``<input
+required="required">``.
+
+To set a boolean attribute, pass ``True`` as the value. ``False`` is also a
+valid value, and causes the attribute to be omitted (this can be useful if you
+set the attribute from a variable):
+
+>>> from minihtml.tags import input
+>>> print(input(required=True, disabled=False))
+<input required="required">
+
 .. _content:
 
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