docs: add note about the nixos service defaulting to sqlite

src/database/database.rs

@@ -38,11 +38,11 @@ pub fn keys(&self) -> impl Iterator<Item = &MapsKey> + Send + '_ { self.maps.key
 
 	#[inline]
 	#[must_use]
-	pub fn is_read_only(&self) -> bool { self.db.secondary || self.db.read_only }
+	pub fn is_read_only(&self) -> bool { self.db.is_read_only() }
 
 	#[inline]
 	#[must_use]
-	pub fn is_secondary(&self) -> bool { self.db.secondary }
+	pub fn is_secondary(&self) -> bool { self.db.is_secondary() }
 }
 
 impl Index<&str> for Database {