Versioning guide (#192)

* copy versioning guide from Go

* format and add para

Author: clickingbuttons

README.md

@@ -22,3 +22,21 @@ but we're open to bug reports and feature requests. Or if you have more general
 feedback, feel free to reach out on
 our [Slack channel](https://polygon.io/contact).
 
+## Release planning
+This client will attempt to follow the release cadence of our API.
+When endpoints are deprecated and newer versions are added, the client will
+maintain two methods in a backwards compatible way
+(e.g. `list_trades` and `list_trades_v4(...)`).
+When deprecated endpoints are removed from the API, we'll rename the versioned
+method (e.g. `list_trades_v4(...)` -> `list_trades(...)`), remove the old method,
+and release a new major version of the client.
+
+The goal is to give users ample time to upgrade to newer versions of our API
+_before_ we bump the major version of the client, and in general, we'll try to
+bundle breaking changes like this to avoid frequent major version bumps.
+
+Exceptions to this are:
+	- Methods under `client.vx`. These are expiremental.
+	- Methods that start with `_*`. We use these internally.
+	- Type annotations. We may modify these based on our JSON responses.
+	- We may add model fields.