Add skeleton spec-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.

spec-conventions.md

@@ -0,0 +1,18 @@
+# Conventions when writing specification text
+
+> **NOTE:** This 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 conventions.
+
+## Spelling, grammar, and typography
+
+## Notation
+
+## Algorithm steps
+
+## Abstract operations
+
+## Implementations of functions and methods
+
+## Normative or editorial?