add some usage info

Author: dnault

README.md

@@ -1,13 +1,52 @@
 # json-skiff
 
-A streaming JSON parser.
+A streaming JSON Pointer locator.
 
 1. Give it a list of JSON pointers and callbacks.
 2. Feed it JSON.
 3. Get notified when the target elements are seen.
 
 Less capable, but simpler and faster than the excellent [JsonSurfer](https://github.com/wanglingsong/JsonSurfer) library.
 
+## Usage
+
+### Selecting array elements
+
+Normally, a `-` in a JSON Pointer refers to the last element of an array.
+`json-skiff` bends the rules a bit, and interprets `-` as matching _all_ array elements.
+
+Consider this JSON document:
+
+```json
+{
+  "magicWords": [
+    "alakazam",
+    "xyzzy"
+  ]
+}
+``` 
+A callback associated with `/magicWords/-` is invoked a total of two times: once for `"alakazam"` and once for `"xyzzy"`. 
+
+The `-` does not need to be the last component of the pointer.
+Consider this JSON document:
+
+```json
+{
+  "widgets": [
+    {
+      "serialNumber": "123"
+    },
+    {
+      "serialNumber": "XYZ"
+    }
+  ]
+}
+```
+
+A callback associated with `/widgets/-/serialNumber` is invoked a total of two times: once for `"123"` and once for `"XYZ"`.
+
+
+## FAQ
 
 ### Is this library an officially supported Couchbase product?