Add PR instruction on screenshot PR

Author: berland

.github/PULL_REQUEST_TEMPLATE.md

@@ -13,6 +13,7 @@ _Short description of the approach_
 - [ ] Commit history is consistent and clean, in line with the [contribution guidelines](https://github.com/equinor/ert/blob/main/CONTRIBUTING.md).
 - [ ] Make sure unit tests pass locally after every commit (`git rebase -i main
       --exec 'just rapid-tests'`)
+- [ ] Review screenshot-PR in ert-testdata if applicable. Merge screenshot-PR in ert-testdata before merging this PR.
 
 ## When applicable
 - [ ] **When there are user facing changes**: Updated documentation