Add sub-heading for additional merge details

Author: jordanmccullough

intermediate/index.md

@@ -373,6 +373,10 @@ $ git branch
 ```
 {% endcapture %}{% include slide-section %}
 
+{% capture slide %}
+### Merge strategies
+{% endcapture %}{% include slide-section %}
+
 {% capture slide %}
 ![diagram](../assets/diagrams/merge-fast-forward-01.svg)
 {% endcapture %}{% include slide-section %}