add mention of search reindexing (#128)

Add a section mentioning when and how to reindex the site search, such as when a new version of the docs is released.

[CHANGELOG_BEGIN]
[CHANGELOG_END]

Author: loiskilberg-da

README.md

@@ -176,6 +176,17 @@ git push
 
 5. Ask on the `#team-daml` Slack channel, mentioning @gary, for someone to help you manually create a snapshot. They will take it from there.
 
+### For each docs release
+
+When there is a change to the root website, the entire site needs to be reindexed for searching. This includes any time a new version of the docs is released. Until the reindexing is done, the site search engine may still bring up deleted and moved pages in search results. Then when a viewer clicks that search result, a 404 error is displayed. Google search will eventually catch up without intervention. Following are the steps to reindex the site search:
+
+1. Log into SiteSearch360 (https://www.sitesearch360.com/).
+2. At the bottom of the sidebar menu, select **Index**.
+3. Scroll down to see a blue button and a red button.
+4. Click the red button for **Empty Entire Index**. It take a couple of minutes to process.
+5. When that has completed, click the blue button for **Re-index All Configured Sources**. This may take about a half hour.
+6. When this has completed, the site search should work as expected.
+
 ## Questions
 
 If you have any questions or comments about these instructions, please reach out to the `#product-docs` channel and/or @katharine/@gary on Slack.