Add document with suggestions for ID fields

Author: DrewML

design-documents/graph-ql/id-fields-object-types.md

@@ -0,0 +1,24 @@
+# ID Fields in Object Types
+
+When a GraphQL Object Type is used to represent an entity that can be referenced by ID, it _should_ follow these best-practices.
+
+## Best Practices
+
+### Do
+
+- Use the `ID` scalar type
+- Use the field name `id` or `_id`
+    - Most GraphQL client libs automatically use this field for caching. Any other field name will require manual caching logic on the client
+        - https://www.apollographql.com/docs/react/caching/cache-configuration/#assigning-unique-identifiers
+        - https://formidable.com/open-source/urql/docs/graphcache/normalized-caching/#key-generation
+- Include an ID field anytime an Object Type _could_ have an ID field
+
+### Do Not
+- Include info in the client-facing description describing how the field is encoded/decoded (should be opaque)
+    - Example: The client shouldn't know if an `ID` is a base64-encoded integer
+- Use `String` or `Int` type
+- Use duplicate IDs across a concrete type. In other words, if the client wants to produce a cache key, the concetenation of a `__typename` + `id` field should _always_ be unique
+
+## Examples where an ID is not helpful
+
+- Wrapper types, like `SearchResultPageInfo`