Add grammar image.

Made with https://jacquev6.github.io/DrawGrammar/

docs/source/Thun.md

@@ -13,7 +13,7 @@ notation, Booleans `true` and `false`, lists enclosed by square brackets
     
     term ::= integer | bool | '[' joy ']' | symbol
     
-    integer ::= [ '-' ] ('0'...'9')+
+    integer ::= 0 | [ '-' ] ('1'...'9') ('0'...'9')*
     
     bool ::= 'true' | 'false'
     
@@ -29,6 +29,9 @@ respective Boolean values.
 That's it.  That's the whole of the grammar.
 
 
+![Thun Grammar Railroad Diagram](./images/grammar.png)
+
+
 ## Types
 
 The original Joy has several datatypes (such as strings and sets)