added small note about project naming

Signed-off-by: Cody Lin <[email protected]>

docs/reference/feature-repository/feature-store-yaml.md

@@ -23,7 +23,7 @@ The following top-level configuration options exist in the `feature_store.yaml`
 * **registry** — Configures the location of the feature registry.
 * **online\_store** — Configures the online store.
 * **offline\_store** — Configures the offline store.
-* **project** — Defines a namespace for the entire feature store. Can be used to isolate multiple deployments in a single installation of Feast.
+* **project** — Defines a namespace for the entire feature store. Can be used to isolate multiple deployments in a single installation of Feast. Should only contain letters, numbers, and underscores.
 
 Please see the [RepoConfig](https://rtd.feast.dev/en/latest/#feast.repo_config.RepoConfig) API reference for the full list of configuration options.