Update PULL_REQUEST_TEMPLATE.md to include visual change reference (#…

…1923)

* Update PULL_REQUEST_TEMPLATE.md

* Change bold to heading markup

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,33 +1,38 @@
-**PR Summary:** 
+### PR Summary: 
 
 <!-- Please include a short description (using non-technical terms, 1-2 sentences) about the changes you are introducing, what problem is being fixed and/or describe the benefit to merchants. This content will be used in our release notes for Dawn on [themes.shopify.com](https://themes.shopify.com/themes/dawn/styles/default#ReleaseNotes). -->
 
 
-**Why are these changes introduced?**
+### Why are these changes introduced?
 
 Fixes #0.
 
-**What approach did you take?**
+### What approach did you take?
 
-**Other considerations**
+### Other considerations
 
-**Decision log**
+### Decision log
 
 | # | Decision | Alternatives | Rationale | Downsides |
 |---|---|---|---|---|
 | 1 |   |   |   |   |
 
-**Testing steps/scenarios**
+
+### Visual impact on existing themes
+<!-- How will this visually affect merchants who upgrade to a new theme version with this change? -->
+
+
+### Testing steps/scenarios
 <!-- List all the testing tasks that applies to your fix to help peers review your work. -->
 - [ ] Step 1
 
-**Demo links**
+### Demo links
 <!-- Please include a link to a demo store that includes preconfigured sections and settings to allow reviewers to easily test the features you are working on. -->
 
 - [Store](url)
 - [Editor](url)
 
-**Checklist**
+### Checklist
 - [ ] Added PR summary for [release notes](https://themes.shopify.com/themes/dawn/styles/default#ReleaseNotes)
 - [ ] Requested review from UX (Only for changes that are affecting the experience or perceivable visual details)
 - [ ] Created a ticket for the [help.shopify.com](https://help.shopify.com) documentation team about updates to theme settings. (Internal-only task)