Add skeleton editorial-conventions.md

This can be a place to put items from
tc39/ecma262#2236 as they are discussed.

I've pre-populated it with h2 headings that roughly match the kinds of
conventions discussed in that issue.

editorial-conventions.md

@@ -0,0 +1,18 @@
+# Editorial conventions when writing specification text
+
+This document describes some of the conventions that we try to follow when writing and editing specification text. Existing text may not follow these conventions exactly, but it should be updated eventually.
+
+> **NB.** The document is currently a skeleton, and should not be considered a complete list.
+> The issue [tc39/ecma262#2236](https://github.com/tc39/ecma262/issues/2236) is a good place to discuss editorial conventions.
+
+## Spelling, grammar, and typography
+
+## Notation
+
+## Algorithm steps
+
+## Abstract operations
+
+## Implementations of functions and methods
+
+## Normative or editorial?