Update README.md with the exercises for the week.

README.md

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 # Materials of the TN2844 course Solid State Physics
 
 ## Lecture notes
@@ -24,7 +23,8 @@
   Exercises 12.3, 12.4, 13.3, 13.4, 14.2
 
 * Week 6: Tight binding and nearly free electrons  
-  (based on chapters 15–16)
+  (based on chapters 15–16)  
+  Exercises 15.1, 15.3, 15.4, 16.1, 16.2
 
 * Week 7: Semiconductors  
   (based on chapters 17–18)