Add a checklist to the PR template (#1693)

We tend to forget adding changelog entries for PRs, so here's an extra reminder

CHANGELOG.md

@@ -19,6 +19,8 @@
  
 ### Chore
 
+ - [#1693](https://github.com/poanetwork/blockscout/pull/1693) - Add a checklist to the PR template
+
 
 ## 1.3.8-beta
 

PULL_REQUEST_TEMPLATE.md

@@ -18,3 +18,20 @@
 ## Upgrading
 
 *If you have any Incompatible Changes in the above Changelog, outline how users of prior versions can upgrade once this PR lands or when reviewers are testing locally.  A common upgrading step is "Database reset and re-index required".*
+
+## Checklist for your PR
+
+<!--
+  Ideally a PR has all of the checkmarks set.
+
+  If something in this list is irrelevant to your PR, you should still set this
+  checkmark indicating that you are sure it is dealt with (be that by irrelevance).
+
+  If you don't set a checkmark (e. g. don't add a test for new functionality),
+  you must be able to justify that.
+-->
+
+  - [ ] I added an entry to `CHANGELOG.md` with this PR
+  - [ ] If I added new functionality, I added tests covering it.
+  - [ ] If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
+  - [ ] I checked whether I should update the docs and did so if necessary