adding docs subtree

docs/step2-09/demo/src/components/TodoHeader.tsx

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+import React from 'react';
+import { Text } from '@uifabric/experiments';
+import { Stack } from 'office-ui-fabric-react';
+import { Pivot, PivotItem, TextField, PrimaryButton } from 'office-ui-fabric-react';
+import { FilterTypes, Store } from '../store';
+import { actionsWithService, actions } from '../actions';
+import { connect } from 'react-redux';
+
+interface TodoHeaderProps {
+  addTodo: (label: string) => void;
+  setFilter: (filter: FilterTypes) => void;
+  filter: FilterTypes;
+}
+
+interface TodoHeaderState {
+  labelInput: string;
+}
+
+class TodoHeader extends React.Component<TodoHeaderProps, TodoHeaderState> {
+  constructor(props: TodoHeaderProps) {
+    super(props);
+    this.state = { labelInput: undefined };
+  }
+
+  render() {
+    return (
+      <Stack gap={10}>
+        <Stack horizontal horizontalAlign="center">
+          <Text variant="xxLarge">todos</Text>
+        </Stack>
+
+        <Stack horizontal gap={10}>
+          <Stack.Item grow>
+            <TextField placeholder="What needs to be done?" value={this.state.labelInput} onChange={this.onChange} />
+          </Stack.Item>
+          <PrimaryButton onClick={this.onAdd}>Add</PrimaryButton>
+        </Stack>
+
+        <Pivot onLinkClick={this.onFilter}>
+          <PivotItem headerText="all" />
+          <PivotItem headerText="active" />
+          <PivotItem headerText="completed" />
+        </Pivot>
+      </Stack>
+    );
+  }
+
+  private onAdd = () => {
+    this.props.addTodo(this.state.labelInput);
+    this.setState({ labelInput: undefined });
+  };
+
+  private onChange = (evt: React.FormEvent<HTMLInputElement>, newValue: string) => {
+    this.setState({ labelInput: newValue });
+  };
+
+  private onFilter = (item: PivotItem) => {
+    this.props.setFilter(item.props.headerText as FilterTypes);
+  };
+}
+
+function mapStateToProps(state: Store) {
+  return { ...state };
+}
+
+function mapDispatchToProps(dispatch: any) {
+  return {
+    addTodo: (label: string) => dispatch(actionsWithService.addTodo(label)),
+    setFilter: (filter: FilterTypes) => dispatch(actions.setFilter(filter))
+  };
+}
+
+const component = connect(
+  mapStateToProps,
+  mapDispatchToProps
+)(TodoHeader);
+
+export { component as TodoHeader };