docs: Document the DISABLE_SIGNUPS config flag. Fixes #8

docs/docs/03-configuration.md

@@ -11,6 +11,7 @@ The app is mainly configured by environment variables. All the used environment
 | REDIS_DB_IDX     | No                                    | Not set   | The db idx to use with redis. It defaults to 0 (in the client) so you don't usually need to set it unless you explicitly want another db. |
 | MEILI_ADDR       | No                                    | Not set   | The address of meilisearch. If not set, Search will be disabled. E.g. (`http://meilisearch:7700`)                                         |
 | MEILI_MASTER_KEY | Only in Prod and if search is enabled | Not set   | The master key configured for meilisearch. Not needed in development environment. Generate one with `openssl rand -base64 36`             |
+| DISABLE_SIGNUPS  | No                                    | false     | If enabled, no new signups will be allowed and the signup button will be disabled in the UI                                               |
 
 ## Inference Configs (For automatic tagging)