Index of lessons updated in README.md

Author: sergiandreplace

README.md

@@ -8,4 +8,7 @@ Remember all the graphical assets and the design is created by Vijay Verma
 
 * Lesson 1: [Planets-Flutter: From design to app](http://sergiandreplace.com/2017/09/planets-flutter-from-design-to-app/)
 * Lesson 2: [Planets-Flutter: creating a Planet card](http://sergiandreplace.com/2017/09/planets-flutter-creating-a-planet-card/)
-* Lesson 3: [Planets-Flutter: adding content to the card](http://sergiandreplace/2017/10/planets-flutter-adding-content-to-the-card/)
+* Lesson 3: [Planets-Flutter: adding content to the card](http://sergiandreplace/2017/10/planets-flutter-adding-content-to-the-card/)
+* Lesson 4: [Planets-Flutter: creating a list of planets](http://sergiandreplace/2017/10/planets-flutter-creating-a-list-of-planets/)
+* Lesson 5: [Planets-Flutter: routing and navigation](http://sergiandreplace/2017/12/planets-flutter-routing-and-navigation/)
+* Lesson 6: [Planets-Flutter: planet detail page](http://sergiandreplace/2018/02/planets-flutter-planet-detail-page//)