Add definitions to the style guide (#1137)

Co-authored-by: Benjie <benjie@jemjie.com>

Author: martinbonnin

STYLE_GUIDE.md

@@ -82,3 +82,12 @@ MyAlgorithm(argOne, argTwo):
     - Let {something} be {true}.
 - Return {something}.
 ```
+
+## Definitions
+
+For important terms, use
+[Spec Markdown definition paragraphs](https://spec-md.com/#sec-Definition-Paragraph).
+
+Definition paragraphs start with `::` and add the matching italicized term to
+the [specification index](https://spec.graphql.org/draft/#index), making it easy
+to reference them.