search: Document search handlers API

packages/gitbook-plugin-search/README.md

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+# plugin-search
+
+This plugin is the interface used by all the search plugins (`plugin-lunr`, `plugin-algolia`, etc.)
+
+## Registering a Search handler
+
+Your plugin must register as a Search handler during its `activate` method:
+
+
+``` js
+GitBook.createPlugin({
+    activate: (dispatch, getState, { Search }) => {
+        dispatch(Search.registerHandler('my-plugin-name', searchHandler));
+    },
+    reduce
+})
+
+/**
+ * Search against a query
+ * @param  {String} query
+ * @return {Promise<List<Result>>}
+ */
+function searchHandler(query, dispatch, getState) {
+    ...
+}
+```
+
+Your search handler must return a List of result-shaped objects. A result object has the following shape:
+
+``` js
+result = {
+  title: string,  // The title of the resource, as displayed in the list of results.
+
+  url:   string,  // The URL to access the matched resource.
+
+  body:  string   // (optional) The context of the matched text (can be a sentence
+                  // containing matching words). It will be displayed near the result.
+}
+```
+
+