While not necessary, submitting a sitemap will make your site appear faster on search indexes.
This Howto covers submitting sitemaps to Google, Bing and Yandex.
Generating the Sitemap
Open http://www.web-site-map.com/ and generate your sitemap. While the site may seem a bit scammy, it works just fine. It will output a sitemap.xml and a broken_links.txt if there are any.
Open the sitemap.xml and change the root site (for example meta.discourse.org)'s frequency to daily and other sites too as you see fit.
Uploading the Sitemap to your Discourse installation
Navigate to your discourse installation’s settings and search for
xml as an authorized extension and click apply.
Navigate to your site assets topic (only visible for staff) and reply to the first post. Click the upload button and upload the previously generated
sitemap.xml. You will see something like
<a class="attachment" href="/uploads/default/17/d86d639b474069e9.xml">sitemap.xml</a> (5.5 KB)
Copy the URL to the sitemap, e.g
/uploads/default/17/d86d639b474069e9.xml and continue with the next step.
Submitting the sitemap
Create accounts at Google, Bing and Yandex and submit the sitemap. Should be self-explanatory, each service has their own help pages.
Where will my site’s pages get found now?
Google, Yahoo, Bing, Yandex, Duckduckgo etc. will now index your site!