Add Notes to style guide.

STYLE.md

@@ -314,6 +314,29 @@ Design Rules
     partial functions.  E.g, if a list must never be empty, represent
     it as a pair of an element and a list instead of just a list.
 
+Notes
+=====
+
+For long comments, we (try to) use the Notes convention from GHC,
+[explained here](https://www.stackbuilders.com/news/the-notes-of-ghc).
+Essentially, instead of writing very long in-line comments that break
+the flow of the code, we write
+
+```
+-- See Note [Foo Bar]
+```
+
+and then somewhere else in the file (perhaps at the bottom), we put
+
+```
+-- Note [Foo Bar]
+--
+-- Here is how you foo the bar...
+```
+
+There is no automation around this or a hard rule for what a "long
+comment" is.  It's just a convention.
+
 Credits
 =======