Document format of additionalMethods

Which is, simple truthy properties. Closes #1, for now.

README.md

@@ -161,7 +161,24 @@ _TBD: what this means for `*asBuffer` options._
 
 ### `additionalMethods` (object)
 
-Format TBD.
+In the form of:
+
+```js
+{
+  foo: true,
+  bar: true
+}
+```
+
+Which says the db has two methods, `foo` and `bar`, that are not part of the `abstract-leveldown` interface. It might be used like so:
+
+```js
+if (db.supports.additionalMethods.foo) {
+  db.foo()
+}
+```
+
+For future extensibility, the properties of `additionalMethods` should be taken as truthy rather than strictly typed booleans. We may add additional metadata (see [#1](https://github.com/Level/supports/issues/1)).
 
 ## Install
 

index.js

@@ -30,17 +30,6 @@ module.exports = function supports () {
     encodings: manifest.encodings || false,
 
     // Methods that are not part of abstract-leveldown or levelup
-    additionalMethods: manifest.additionalMethods || {
-      // May look something like:
-      // approximateSize: {
-      //   sync: false,
-      //   callback: true,
-      //   promise: false
-      // },
-      // createWriteStream: {
-      //   sync: true,
-      //   writable: true
-      // }
-    }
+    additionalMethods: manifest.additionalMethods || {}
   })
 }