Add a pull request template (#1)

Add a pull request template

### Motivation

To help maintain consistency in pull request descriptions, and encourage PR authors to provide enough information to reviewers upfront, let's add a PR template to the repo.

### Modifications

Added a PR template file based on the SwiftNIO one, with slight modifications (removed colons in headings, added a test plan section). Adjusted the NOTICE file to reflect that.

### Result

PR authors will be presented with the template when creating a new PR using GitHub's UI or tools that support templates.

### Test Plan

Well, this PR is already being open using the template :)


Reviewed by: simonjbeaumont

Builds:
     ✔︎ pull request validation (5.8) - Build finished. 
     ✔︎ pull request validation (5.9) - Build finished. 
     ✔︎ pull request validation (nightly) - Build finished. 
     ✔︎ pull request validation (soundness) - Build finished. 

#1

Author: czechboy0

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,15 @@
+### Motivation
+
+_[Explain here the context, and why you're making that change. What is the problem you're trying to solve.]_
+
+### Modifications
+
+_[Describe the modifications you've made.]_
+
+### Result
+
+_[After your change, what will change.]_
+
+### Test Plan
+
+_[Describe the steps you took, or will take, to qualify the change - such as adjusting tests and manual testing.]_

NOTICE.txt

@@ -26,7 +26,7 @@ components that this product depends on.
 
 -------------------------------------------------------------------------------
 
-This product contains derivations of various scripts from SwiftNIO.
+This product contains derivations of various scripts and templates from SwiftNIO.
 
   * LICENSE (Apache License 2.0):
     * https://www.apache.org/licenses/LICENSE-2.0

scripts/check-license-headers.sh

@@ -42,6 +42,7 @@ read -ra PATHS_TO_CHECK_FOR_LICENSE <<< "$( \
   ":(exclude).gitignore" \
   ":(exclude).spi.yml" \
   ":(exclude).swift-format" \
+  ":(exclude).github/*" \
   ":(exclude)CODE_OF_CONDUCT.md" \
   ":(exclude)CONTRIBUTING.md" \
   ":(exclude)CONTRIBUTORS.txt" \