Updated Style Guide for Nim Code (markdown)

Style-Guide-for-Nim-Code.md

@@ -131,6 +131,10 @@ changed in the future.
   variables remain immutable, and gives those who read the code a better idea
   of the code's purpose.
 
+- For new types, it is usually recommended to have both 'ref' and 'object'
+  versions of the type available for others to use. By making both variants
+  available for use, the type may be allocated both on the stack and the heap.
+
 
 ### Conventions for multi-line statements and expressions ###
 - Any tuple type declarations that are longer than one line should use the