Update PULL_REQUEST_TEMPLATE.md

Signed-off-by: Jan-Otto Kröpke <mail@jkroepke.de>

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,14 +1,19 @@
-**What this PR does / why we need it**:
+#### What this PR does / why we need it
 
-**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
+#### Which issue this PR fixes
+*(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
 
-**Special notes for your reviewer**:
+- fixes #
+
+#### Special notes for your reviewer
 
-**Particularly user-facing changes**:
+#### Particularly user-facing changes
 
-**PR Readiness Checklist**:
+#### Checklist
 
 Complete these before marking the PR as `ready to review`:
 
+<!-- [Place an '[x]' (no spaces) in all applicable fields.] -->
+- [ ] [DCO](https://github.com/prometheus-community/helm-charts/blob/main/CONTRIBUTING.md#sign-off-your-work) signed
 - [ ] The PR title has a summary of the changes
 - [ ] The PR body has a summary to reflect any significant (and particularly user-facing) changes introduced by this PR