Add section headings

README.md

@@ -28,9 +28,7 @@ rather than relying upon userland libraries.
 - The primary use case is the retrieval of localized text ("a message")
  given a message identifier and a previously specified locale.
 
-## Description
-
-### API
+## API Description
 
 The MF2 specification is still being developed by the working group.
 The API below is based upon one proposal under consideration,
@@ -39,6 +37,8 @@ In particular, the API shapes of
 `MessageFormatOptions`, `MessageData`, `MessageResourceData`, and `ResolvedMessageFormatOptions`
 will depend upon the data model chosen by the working group.
 
+### MessageData and MessageResourceData
+
 The `MessageData` and `MessageResourceData` interfaces will be defined by
 the MF2 data model developed by the MF2 working group.
 `MessageData` contains a parsed representation of a single message for a particular locale.
@@ -50,6 +50,8 @@ interface MessageData {}
 interface MessageResourceData {}
 ```
 
+### MessageResource
+
 A `MessageResource` is a group of related messages for a single locale.
 Messages can be organized in a flat structure, or in hierarchy, using paths.
 Conceptually, it is similar to a file containing a set of messages,
@@ -74,6 +76,8 @@ interface MessageResource {
 }
 ```
 
+### MessageFormat
+
 The `Intl.MessageFormat` constructor creates `MessageFormat` instances for a given locale,
 `MessageFormatOptions` and an optional set of `MessageResource`s.
 The remaining operations are defined on `MessageFormat` instances.
@@ -104,6 +108,8 @@ interface MessageFormat {
 }
 ```
 
+#### Constructor options and resolvedOptions()
+
 The interfaces for
 `MessageFormatOptions` and `ResolvedMessageFormatOptions`
 will depend on the final MF2 data model.
@@ -137,6 +143,8 @@ type MessageFormatterFunction = (
 ) => MessageValue
 ```
 
+#### format() and getMessage()
+
 For formatting a message, two methods are provided: `format()` and `getMessage()`.
 The first of these will always return a simple string,
 while the latter returns a `ResolvedMessage` object or `undefined` if the message was not found.
@@ -151,6 +159,8 @@ These methods have the following arguments:
  If `onError` is not defined,
  errors will be ignored and a fallback representation used for the corresponding message part.
 
+### MessageValue
+
 `ResolvedMessage` is intended to provide a building block for the localization of messages
 in contexts where its representation as a plain string would not be sufficient.
 The `source` of a `MessageValue` provides an opaque identifier for the value,