Add D_Inbuilt_Gatherers.md

Author: ZahidFKhan

src/test/java/com/github/streams/learn/b_advanced/gatherers/C_When to use Custom gatherer.md

@@ -1,4 +1,6 @@
-When to use Custom Gatherers?
+## When to use Custom Gatherers?
+
+You should avoid using custom gatherers unless you're required to do so.
 
 1. When you're not able to solve the problem using filter, map or flatMap.
 2. When there exists no solution in the Gatherers even in latest JDK.

src/test/java/com/github/streams/learn/b_advanced/gatherers/D_Inbuilt_Gatherers.md

@@ -0,0 +1,3 @@
+## Inbuilt Gatherers?
+
+1.