Updating changelog for 2.0.0 beta (#108)

Author: aliabbasrizvi

CHANGELOG.md

@@ -1,3 +1,36 @@
+## 2.0.0b1
+March 29th, 2018
+
+This beta release introduces APIs for Feature Management. It also introduces some breaking changes listed below.
+
+### New Features
+- Introduced the `is_feature_enabled` API to determine whether to show a feature to a user or not.
+```
+is_enabled = optimizel_client.is_feature_enabled('my_feature_key', 'my_user', user_attributes)
+```
+
+- All enabled features for the user can be retrieved by calling:
+```
+enabled_features = optimizely_client.get_enabled_features('my_user', user_attributes)
+```
+
+- Introduced Feature Variables to configure or parameterize a feature. There are four variable types: `String`, `Integer`, `Double`, `Boolean`.
+```
+string_variable = optimizely_client.get_feature_variable_string('my_feature_key', 'string_variable_key', 'my_user')
+integer_variable = optimizely_client.get_feature_variable_integer('my_feature_key', 'integer_variable_key', 'my_user')
+double_variable = optimizely_client.get_feature_variable_double('my_feature_key', 'double_variable_key', 'my_user')
+boolean_variable = optimizely_client.get_feature_variable_boolean('my_feature_key', 'boolean_variable_key', 'my_user')
+```
+
+### Breaking changes
+- The `track` API with revenue value as a stand-alone parameter has been removed. The revenue value should be passed in as an entry in the event tags dict. The key for the revenue tag is `revenue` and the passed in value will be treated by Optimizely as the value for computing results.
+```
+event_tags = {
+  'revenue': 1200
+}
+
+optimizely_client.track('event_key', 'my_user', user_attributes, event_tags)
+
 ## 1.4.0
 - Added support for IP anonymization.
 - Added support for notification listeners.