improved docs

docs/manual/common-questions.md

@@ -0,0 +1,55 @@
+# Common Questions
+
+## What does attempt to index a nil value mean!?
+It means you tried to access a field of something that doesn't exist, for example:
+```lua
+local thisIsNil = nil
+print(thisIsNil.what)
+print(thisIsNil.something())
+```
+
+## Why i'm getting "cannot access field test of userdata<something>" errors?
+Because you are trying to access a field in a C# class that doesn't exist.
+```lua
+print(Item.thisDoesntExist)
+```
+
+## I'm getting a super big error with things related to the C# side
+It usually happens when you call something on the C# side and you provide nil inputs, you can get a better idea of it by analyzing the error message and trying to link it your lua code, for example:
+```lua
+Game.SendMessage(nil)
+```
+
+This will result in 
+```
+[LUA ERROR] System.NullReferenceException: Object reference not set to an instance of an object.
+at Barotrauma.Networking.ChatMessage.GetChatMessageCommand(String message, String& messageWithoutCommand)
+at Barotrauma.Networking.GameServer.SendChatMessage(String message, Nullable`1 type, Client senderClient, 
+Character senderCharacter, PlayerConnectionChangeType changeType)
+```
+You can easily tell that the error has something to do with chat messages, and by looking back at your Lua code you can easily see whats causing it.
+
+## How do i list all clients, characters and items?
+```lua
+for _, client in pairs(Client.ClientList) do
+
+end
+
+for _, character in pairs(Character.CharacterList) do
+
+end
+
+for _, item in pairs(Item.ItemList) do
+
+end
+```
+
+## Running pairs() on an enumerator doesn't work!
+pairs() Returns an enumerator that iterates through the entire table keys, if you already have an enumerator, you can just pass it in directly.
+```lua
+-- get first item ever created and loop through all the values
+for item in Item.ItemList[1].OwnInventory.AllItems do
+
+end
+```
+