Update README.md

Added instructions for configuring as a replica set.

README.md

@@ -26,7 +26,7 @@ Before anything else you will need to install the application dependencies:
 - **ElasticSearch 7.17.7** https://www.elastic.co/downloads/past-releases/elasticsearch-7-17-6 Please note that ElasticSearch requires Java. Follow the instructions to install the package manually, you also probably need to disable ml module in the ElasticSearch config file:
   `xpack.ml.enabled: false`
 - **ICU Analysis Plugin (recommended)** [installation](https://www.elastic.co/guide/en/elasticsearch/plugins/current/analysis-icu.html#analysis-icu) Adds support for number sorting in texts and solves other language sorting nuances. This option is activated by setting the env var USE_ELASTIC_ICU=true before running the server (defaults to false/unset).
-- **MongoDB 6.0** https://www.mongodb.com/docs/v5.0/installation/ . If you have a previous version installed, please follow the instructions on how to [upgrade here](https://www.mongodb.com/docs/manual/release-notes/5.0-upgrade-standalone/). The new mongosh dependency needs to be added [installation](https://www.mongodb.com/docs/mongodb-shell/).
+- **MongoDB 6.0** https://www.mongodb.com/docs/v5.0/installation/ . The MongoDB installation needs to be configured as a Replica Set.  It can be a single-node replica set, but Replica Set must be [initialized](https://www.mongodb.com/docs/v6.0/tutorial/deploy-replica-set/).  If you have a previous version installed, please follow the instructions on how to [upgrade here](https://www.mongodb.com/docs/manual/release-notes/5.0-upgrade-standalone/). The new mongosh dependency needs to be added [installation](https://www.mongodb.com/docs/mongodb-shell/).
 - **Yarn** https://yarnpkg.com/en/docs/install.
 - **pdftotext (Poppler)** tested to work on version 0.86 but it's recommended to use the latest available for your platform https://poppler.freedesktop.org/. Make sure to **install libjpeg-dev** if you build from source.