Instruct to create associated issue in PR template. (#1624)

We use issues for tracking work and new features in RMM. This adds instruction to the PR template to suggest contributors should create an associated issue if it doesn't already exist.

Fixes #1623

Authors:
  - Mark Harris (https://github.com/harrism)

Approvers:
  - James Lamb (https://github.com/jameslamb)

URL: #1624

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,4 +1,8 @@
 ## Description
+<!-- ALL RMM PULL REQUESTS SHOULD HAVE AN ASSOCIATED ISSUE -->
+<!-- We use issues for tracking work and features in RMM. If no issue exists for this PR, first -->
+<!-- create a new issue. -->
+
 <!-- Provide a standalone description of changes in this PR. -->
 <!-- Reference any issues closed by this PR with "closes #1234". -->
 <!-- Note: The pull request title will be included in the CHANGELOG. -->