Document json.null

Author: well-in-that-case

docs/Runtime Environment/Library Reference/JSON.md

@@ -67,4 +67,7 @@ decoded = json.decode(encoded)
 assert(decoded.key == "Hello")
 assert(type(decoded) == "table")
 assert(decoded.nested.nested_key == 1337)
-```
+```
+---
+## Encoding Null Values
+The `json.null` value — assuming you did `local json = require("json")` — can be used to encode null JSON values.