adding documentation

docs/step1-06/README.md

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+## demo
+
+add state to AppTodo
+pass to header and list
+
+### header
+
+- pull filter from props
+- update buttons with classNames for active
+- Add value/onChange to input
+
+### TodoList
+
+- write map to loop through items, pass key and todos
+
+## exercise
+
+update footer to include todos
+add item count and item(s)
+update ListItem, pull in props, use label/completed already passed in