added an accessibility checkbox in the pull request template

Author: Izzy-Snyder

.github/PULL_REQUEST_TEMPLATE.md

@@ -8,3 +8,4 @@ I have verified that this pull request:
 * [ ] has no test errors (`npm run test`)
 * [ ] is from a uniquely-named feature branch and is up to date with the  `develop` branch.
 * [ ] is descriptively named and links to an issue number, i.e. `Fixes #123`
+* [ ] meets the standards outlined in the [accessibility guidelines](https://github.com/processing/p5.js-web-editor/blob/develop/contributor_docs/accessibility.md)