Add initial best practice decision record

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Pa11y testing results

``` > development-guide@1.0.0 pa11y-ci:sitemap > pa11y-ci --sitemap https://engineering.18f.gov/sitemap.xml --sitemap-find https://engineering.18f.gov --sitemap-replace http://localhost:4000 --sitemap-exclude "/*.pdf" Running Pa11y on 43 URLs: > http://localhost:4000/architecture-reviews/ - 0 errors > http://localhost:4000/about/ - 0 errors > http://localhost:4000/accessibility-scanning/ - 0 errors > http://localhost:4000/apis/ - 0 errors > http://localhost:4000/books-we-have-read/ - 0 errors > http://localhost:4000/browser-testing/ - 0 errors > http://localhost:4000/architecture-reviews/data-act-pilot/ - 0 errors > http://localhost:4000/architecture-reviews/micro-purchase/ - 0 errors > http://localhost:4000/datastore-selection/ - 0 errors > http://localhost:4000/code-review/ - 0 errors > http://localhost:4000/continuous-deployment/ - 0 errors > http://localhost:4000/css/ - 0 errors > http://localhost:4000/example-workflows/ - 0 errors > http://localhost:4000/docker/ - 0 errors > http://localhost:4000/development-environments/ - 0 errors > http://localhost:4000/frontend/ - 0 errors > http://localhost:4000/incident-reports/ - 0 errors > http://localhost:4000/incident-reports/cloud-gov/ - 0 errors > http://localhost:4000/ - 0 errors > http://localhost:4000/integrations/ - 0 errors > http://localhost:4000/language-selection/ - 0 errors > http://localhost:4000/javascript/ - 0 errors > http://localhost:4000/license/ - 0 errors > http://localhost:4000/laptop-setup/ - 0 errors > http://localhost:4000/markdown/ - 0 errors > http://localhost:4000/people/ - 0 errors > http://localhost:4000/people/assessment/ - 0 errors > http://localhost:4000/nodejs/ - 0 errors > http://localhost:4000/on-call/ - 2 errors > http://localhost:4000/project-setup/ - 0 errors > http://localhost:4000/release-strategies/ - 0 errors > http://localhost:4000/resources/ - 0 errors > http://localhost:4000/ruby/ - 0 errors > http://localhost:4000/python/ - 0 errors > http://localhost:4000/security/ - 0 errors > http://localhost:4000/security/cloud-services/ - 0 errors > http://localhost:4000/security/dependency-remediation/ - 0 errors > http://localhost:4000/security/content-security-policy/ - 0 errors > http://localhost:4000/security/output-encoding/ - 0 errors > http://localhost:4000/security/incident-response-drills/ - 0 errors > http://localhost:4000/sharepoint/ - 0 errors > http://localhost:4000/workflow/ - 0 errors > http://localhost:4000/web-architecture/ - 0 errors Errors in http://localhost:4000/on-call/: • ARIA role must be appropriate for the element (https://dequeuniversity.com/rules/axe/3.5/aria-allowed-role?application=axeAPI) (#fnref:1)