Add Elasticsearch Instructions to README

Some of the changes required to successfully switch from using OpenSearch to Elasticsearch are undocumented. This has caused people issues so let's document the steps required to make the switch as easy as possible.

README.md

@@ -216,6 +216,12 @@ bin/restart
 open https://magento.test
 ```
 
+### Elasticsearch vs OpenSearch
+OpenSearch is set as the default search engine when setting up this project. Follow the instructions below if you want to use Elasticsearch instead:
+1. Comment out or remove the `opensearch` container in both the [`compose.yaml`](https://github.com/markshust/docker-magento/blob/master/compose/compose.yaml#L55-L66) and [`compose.healthcheck.yaml`](https://github.com/markshust/docker-magento/blob/master/compose/compose.healthcheck.yaml#L38-L43) files
+2. Uncomment the `elasticsearch` container in both the [`compose.yaml`](https://github.com/markshust/docker-magento/blob/master/compose/compose.yaml#L70-L81) and [`compose.healthcheck.yaml`](https://github.com/markshust/docker-magento/blob/master/compose/compose.healthcheck.yaml#L45-L50) files
+3. Update the `bin/setup` command to use the [`$ES_HOST` variable as the value for the `--elasticsearch-host` argument passed to `setup:install`](https://github.com/markshust/docker-magento/blob/master/compose/bin/setup#L65)
+
 ## Updates
 
 To update your project to the latest version of `docker-magento`, run: