add tone of voice

Author: martinbonnin

STYLE_GUIDE.md

@@ -82,3 +82,16 @@ MyAlgorithm(argOne, argTwo):
     - Let {something} be {true}.
 - Return {something}.
 ```
+
+## Tone of voice
+
+The GraphQL specification is a reference document and should use neutral and
+descriptive tone of voice.
+
+Favor the present tense. The present tense is usually clearer and shorter:
+
+✅ Present: The client then sends a request to the server.
+❌ Future: The client will then send a request to the server.
+
+Avoid repetition. Repetition adds more cognitive load and possibilities for different
+parts of the specification to diverge in their meaning.