Update the pr template (#1862)

Add "Why is this needed".

I can get a description of what the PR does from the code changes,
answering the why question is more important.

.github/PULL_REQUEST_TEMPLATE.md

 <!--
 Thank you for contributing to CoreDNS!
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 Please provide the following information to help us make the most of your pull request:
 -->
 
-### 1. What does this pull request do?
-
+### 1. Why is this pull request needed and what does it do?
 
 ### 2. Which issues (if any) are related?
 
-
 ### 3. Which documentation changes (if any) need to be made?
-