added stack navigator

Created new branch, seems to be working fine.

src/components/screens/Modify/modify.js

@@ -0,0 +1,17 @@
+export default function modify() {
+  return (
+    <View style={styles.container}>
+      <Text>activity 1 complete on week 1 yay!</Text>
+      <StatusBar style="auto" />
+    </View>
+  );
+}
+
+const styles = StyleSheet.create({
+  container: {
+    flex: 1,
+    backgroundColor: "#fff",
+    alignItems: "center",
+    justifyContent: "center",
+  },
+});