Add intro.execution C++17 index item.

jaredhoberock · jaredhoberock · commit a1ffcaec4842 · 2017-11-15T18:25:23.000-06:00
diff --git a/cxx17_index.json b/cxx17_index.json
@@ -1,4 +1,5 @@
 {
+    "intro.execution": "4.6",
     "library": "20"
 }
 
