content/generics-next-step: add Introduction section marker

Change-Id: I33e23b3ed79137fb296dfeccec83baef2e30bc73
Reviewed-on: https://go-review.googlesource.com/c/blog/+/238005
Reviewed-by: Robert Griesemer <gri@golang.org>

content/generics-next-step.article

diff --git a/content/generics-next-step.article b/content/generics-next-step.article
index 5523f0a..8ae6161 100644
--- a/content/generics-next-step.article
+++ b/content/generics-next-step.article
@@ -6,6 +6,8 @@
 Ian Lance Taylor
 Robert Griesemer
 
+## Introduction
+
 It’s been almost a year since we [last wrote about the possibility of
 adding generics to Go](https://blog.golang.org/why-generics).
 It’s time for an update.