chore: v6 migration guide (#328)

README.md

@@ -5,6 +5,11 @@
 
 This is the Python client for [Unleash](https://github.com/unleash/unleash).  It implements [Client Specifications 1.0](https://docs.getunleash.io/client-specification) and checks compliance based on spec in [unleash/client-specifications](https://github.com/Unleash/client-specification)
 
+>  **Migrating to v6**
+>
+> If you use custom strategies or access the `features` property on the Unleash Client, read the complete [migration guide](./v6_MIGRATION_GUIDE.md) before upgrading to v6.
+
+
 What it supports:
 * Default activation strategies using 32-bit [Murmurhash3](https://en.wikipedia.org/wiki/MurmurHash)
 * Custom strategies

v6_MIGRATION_GUIDE.md

@@ -0,0 +1,62 @@
+# Migrating to Unleash-Client-Python 6.0.0
+
+This guide highlights the key changes you should be aware of when upgrading to v6.0.0 of the Unleash client.
+
+## Removed direct access to feature flags
+
+Direct access to the feature flag objects through `UnleashClient.features` has been removed. All classes related to the internal representation of feature flags are no longer publicly accessible in the SDK.
+
+The SDK now provides an `UnleashClient.feature_definitions()` method, which returns a list of feature flag names, their type, and the project they're bound to.
+
+## Changes to custom strategies
+
+Custom strategies have undergone some changes that require updates to their implementations. This is a strict requirement: any strategy that does not implement the correct interface will throw an exception at startup.
+
+The interface changes are as follows:
+
+- Strategies no longer inherit from a base class.
+- The apply method now accepts a second parameter, `parameters`. In legacy versions, this functionality was managed by the `load_provisioning()` method.
+
+Here is an example of a legacy strategy:
+
+``` python
+class CatStrategy(Strategy):
+    def load_provisioning(self) -> list:
+        return [x.strip() for x in self.parameters["sound"].split(",")]
+
+    def apply(self, context: dict = None) -> bool:
+        default_value = False
+
+        if "sound" in context.keys():
+            default_value = context["sound"] in self.parsed_provisioning
+
+        return default_value
+```
+
+This is now written as:
+
+``` python
+class CatStrategy:
+    def apply(self, parameters: dict, context: dict = None) -> bool:
+        default_value = False
+
+        parsed_parameters = [x.strip() for x in parameters["sound"].split(",")]
+
+        if "sound" in context.keys():
+            default_value = context["sound"] in parsed_parameters
+
+        return default_value
+
+```
+
+Strategies are now mounted as an instance rather than a class object when configuring the SDK:
+
+``` python
+
+custom_strategies_dict = {"amIACat": CatStrategy()}
+
+unleash_client = UnleashClient(
+    "some_unleash_url", "some_app_name", custom_strategies=custom_strategies_dict
+)
+
+```