Add recommendation to preserve order where possible

Author: benjie

spec/Appendix A -- Notation Conventions.md

@@ -247,3 +247,11 @@ entry has a unique key, and can be directly referenced by that key.
 :: An _ordered map_ is a map which has a defined order. An entry added to an
 ordered map, which does not have an entry with that key, is ordered after
 existing entries.
+
+**Preserving order**
+
+To improve legibility, when possible implementations should preserve observable
+order for unordered data collections. For example, if applying a grammar to an
+input string produces an unordered set, serializing that set (to a string or
+other observable output) should produce the same order found in the original
+input string.

spec/Section 4 -- Introspection.md

@@ -117,6 +117,17 @@ Tools built using GraphQL introspection should respect deprecation by
 discouraging deprecated use through information hiding or developer-facing
 warnings.
 
+**Ordering**
+
+When possible, implementations should preserve observable order for unordered
+data collections. For example, if a schema was produced from a source
+{TypeSystemDocument} then introspection of fields, input fields, arguments, enum
+values, directives, union members, implemented interfaces and so on should
+produce the same order as found in the source.
+
+Note: This recommendation is to improve legibility and stability of schema
+representations.
+
 **Schema Introspection Schema**
 
 The schema introspection system is itself represented as a GraphQL schema. Below