Added a section about projections

Detailed instructions on how to use a projection.

README.md

@@ -123,14 +123,64 @@ The function takes two parameters: the request object and the response object. T
 
 For instance, you can use a filter to display only results for a particular user: 
 
-```
+```javascript
 var filterUser = function(req, res) {
  return {user: req.user};
 }
 
 var notes = restifyMongoose(Note, {filter: filterUser});
 ```
 
+## Projection
+
+A projection is a function, used by the `query` and `detail` opersations, which takes the request object, the result model, and a callback. This function should invoke the callback exactly once. This callback takes an error and a model item as it's two parameters. Use `null` for the error is there is no error.
+
+For instance, the default detail and list projections are as follows:
+
+```javascript
+function (req, item, cb) {
+ cb(null, item);
+};
+```
+
+A projection is useful if you need to manipulate the result item before returning it in the response. For instance, you may not want to return the passwordHash for a User data model.
+
+```javascript
+// If this is the schema
+var UserSchema = new Schema({
+ username: String,
+ email: String,
+ passwordHash: String
+});
+
+// This is a projection translating _id to id and not including passwordHash
+var userProjection = function(req, item, cb) {
+ var user = {
+ id: item._id,
+ username: item.username,
+ email: item.email
+ };
+ cb(null, user);
+};
+```
+
+Projection functions are specified in the options for the resitfy-mongoose contructor, the query function, or the detail function.
+
+For the construtor, the options are `listProjection` and `detailProjection`
+
+```javascript
+var users = restifyMongoose(User, {listProjection: userProjection, detailProjection: userProjection});
+users.serve('/users', restifyServer);
+```
+
+For both query and detail, the option is `projection`
+var users = restifyMongoose(User);
+
+```javascript
+users.detail({projection: userProjection});
+users.query({projection: userProjection});
+```
+
 ## Changelog
 
 ### 0.2.0