Add SessionToken changelog

Author: bartelink

CHANGELOG.md

@@ -11,6 +11,8 @@ The `Unreleased` section name is replaced by the expected version of next releas
 ### Added
 
 - `Stream.TransactAsyncEx`, exposing the `Core.ISyncContext` at conclusion of the sync operation, affording the ability to examine the post-state `Version` etc. (This paves the way for exposing [`SessionToken`](https://github.com/jet/equinox/issues/192) at a later point without a breaking change) [#194](https://github.com/jet/equinox/pull/194)
+- `Core.ISyncContext.SessionToken`: Allows one to inspect the `SessionToken` for relevant stores (i.e., `CosmosDB`), after loading and/or syncing has concluded [#195](https://github.com/jet/equinox/pull/195) 
+
 
 ### Changed