add doc for string.boolean

docs/string.rst

@@ -83,3 +83,22 @@ String
     >>> from pydu.string import sort
     >>> sort('dabc')
     'abcd'
+
+
+.. py:function:: pydu.string.boolean(obj)
+
+    Convert obj to a boolean value.
+
+    If obj is string, obj will converted by case-insensitive way:
+
+        * convert `yes`, `y`, `on`, `true`, `t`, `1` to True
+        * convert `no`, `n`, `off`, `false`, `f`, `0` to False
+        * raising TypeError if other values passed
+
+    If obj is non-string, obj will converted by ``bool(obj)``.
+
+    >>> from pydu.string import boolean
+    >>> boolean('yes')
+    True
+    >>> boolean('no')
+    False