Added point about types to PR checklist (#920)

.github/PULL_REQUEST_TEMPLATE.md

@@ -5,5 +5,6 @@ Please explain the changes you made here.
 - [ ] Format code according to the existing code style (run `npm run lint:check -- CHANGED_FILES` and fix problems in changed code)
 - [ ] Create tests which fail without the change (if possible)
 - [ ] Make all tests (unit and integration) pass (`npm run test:unit` and `npm run test:integration`)
+- [ ] Extend the types in index.d.ts file (if necessary)
 - [ ] Extend the README / documentation and ensure is properly displayed (if necessary)
 - [ ] Provide JIRA issue id (if possible) or GitHub issue id in commit message