Add information about tables, thanks omarroth

About-the-database-and-tables.md

@@ -1,10 +1,10 @@
 Invidious needs one PostgreSQL database with five tables.
 
-`channel_videos`  
-`channels`  
-`nonces`  
-`users`  
-`videos`  
+`channel_videos` Stores truncated video info, used to create user feeds  
+`channels` Stores UCID and author name  
+`nonces` Keeps track of tokens issued to prevent CSRF  
+`users` Stores user info, such as preferences, username, subscriptions  
+`videos` Stores video cache, used to create "top" page  
 
 The table `videos` grows a lot and needs the most storage. You can clean it up using following commands:
 ```bash