Add Pull Request templates (#1086)

This fix adds a Pull Request templates to be used.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

.github/PULL_REQUEST_TEMPLATE.md

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