Update PR template with warning (#869)

Author: Larah Armstrong

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,3 +1,5 @@
+#### **WARNING**: Please make your PR against the `develop` branch.
+- - -
 **Select the type of change:**
 <!-- What types of changes does your code introduce? Put an `x` in all the boxes that apply: -->
 - [ ] Small Changes - Typos, formatting, slight revisions
@@ -7,7 +9,5 @@
 **Purpose of the Pull Request:**
 <!-- Describe what the PR fixes or adds. -->
 
-
-**Issue Number:** 
-<!-- Post the issue or ticket number addressed by the PR. -->
-
+**Issue Number:**
+<!-- Post the issue or ticket number addressed by the PR. -->