Add Git instructions

notes-and-examples/2026.06.12/overview.md

@@ -3,3 +3,23 @@
 - Write a red-black tree with rebalancing
 - Assignment: finish the red-black tree
 - Next week: introduce graphs and graph algorithms
+
+## Git reference
+
+Add all new files and changes to your code:
+
+```
+git add .
+```
+
+Commit with a message:
+
+```
+git commit -m "Hello World"
+```
+
+Push changes:
+
+```
+git push
+```