Make field-subheadings as "<h2>" to make them a little easier to find.

README.md

@@ -452,7 +452,7 @@ Pretty-printed:
 }
 ```
 
-Core fields:
+## Core fields
 
 - `v`: Required. Integer. Added by Bunyan. Cannot be overridden.
   This is the Bunyan log format version (`require('bunyan').LOG_VERSION`).
@@ -486,7 +486,7 @@ well. This is why we're using JSON. Some suggestions and best practices
 follow (feedback from actual users welcome).
 
 
-Recommended/Best Practice Fields:
+## Recommended/Best Practice Fields
 
 - `err`: Object. A caught JS exception. Log that thing with `log.info(err)`
     to get:
@@ -539,7 +539,7 @@ Recommended/Best Practice Fields:
     }
     ```
 
-Other fields to consider:
+## Other fields to consider
 
 - `req.username`: Authenticated user (or for a 401, the user attempting to
   auth).