Add locally building and testing to the PR template

.github/pull_request_template.md

@@ -2,6 +2,7 @@
 
 - [ ] I have read the [App Requirements][reqs] and [App Maintenance][maint] pages.
 - [ ] My pull request follows the instructions at [App Submission][submission].
+- [ ] I have [built][build] and tested the submission locally.
 - [ ] I am using only the minimal set of permissions. *(If not, please explain each non-standard permission.)*
 - [ ] All assets referenced in the manifest are redistributable by any party.  If not, the unredistributable parts are using an extra-data source type.
 - [ ] I am an upstream contributor to the project. If not, I contacted upstream developers about submitting their software to Flathub. **Link:**
@@ -11,3 +12,4 @@
 [reqs]: https://docs.flathub.org/docs/for-app-authors/requirements
 [maint]: https://docs.flathub.org/docs/for-app-authors/maintanance
 [submission]: https://docs.flathub.org/docs/for-app-authors/submission
+[build]: https://docs.flatpak.org/en/latest/first-build.html