Documentation for versioning; update changelog

Author: autopulated

changelog.md

@@ -1,3 +1,8 @@
+## 1.3.0
+ * Adds support for model versioning, with associated schema fragment to name
+   the version field. To disable versioning pass options.versioning: false to
+   the Schema constructor.
+
 ## 1.2.0
  * Support $between and $begins query operators
  * Improved error messages for unsupported queries

readme.md

@@ -269,6 +269,7 @@ Supported options:
    Documents](#indexing-documents) for details.
  * `options.generateId`: A function used to generate a new id for documents of
    this type. Defaults to ``` () => `${schema.name}.${new ObjectId()}` ```
+ * `options.versioning`: Pass `false` to disable [versioning](#document-versioning) for instances of this schema.
 
 After creating a schema, [`.methods`](#schemamethods),
 [`.statics`](#schemastatics), [`.virtuals`](#schemavirtuals), and
@@ -364,6 +365,11 @@ Special fields are defined by using fragments of schema by value.
  * `DynamoDM().TypeField`: used to indicate the type field, which stores the
    name of the model that a saved document was created with. The default type
    field name is `type`.
+ * `DynamoDM().VersionField`: The version field, which stores a number that is
+   incremented by 1 each time a model is saved, and is used to prevent data
+   from being silently overwritten by multiple clients accessing the same document.
+   The default version field name is `v`. See [document
+   versioning](#document-versioning) for details.
  * `DynamoDM().CreatedAtField`: used to indicate a timestamp field that is
    updated when a model is first created by dynamodm. This field is not used
    unless you include this schema fragment in a model's schema.
@@ -632,6 +638,18 @@ await aComment.save();
 const stingified = JSON.stringify(await aComment.toObject());
 ```
 
+### Document Versioning
+The [version field](#built-in-schema-fragments) of a model is incremented each
+time it is saved, starting at 0 for un-saved models. the `.save()` and
+`.remove()` methods check that the version in the database is the same as the
+current one using a [Condition
+Expression](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.ConditionExpressions.html)
+before updating or deleting the data (they fail with an error if the version
+does not match).
+
+Versioning can be disabled for a model by setting the [Schema's
+`options.versioning` property](#schema-options) to `false`.
+
 ## Getting Documents by ID
 ### static async Model.getById(id)
 Get a document by its ID. By default models use `.id` as the ID field. It's