Set CHANGELOG.md for 0.26.0 release

Until we have some process built around this file, it will become stale easily. For now, let's manually note the user facing changes in 0.26.0.

Author: awalker4

CHANGELOG.md

@@ -1,8 +1,12 @@
-## 0.26.0-dev
+## 0.26.0
 
 ### Enhancements
+* Switch to a httpx based client instead of requests
+* Switch to poetry for dependency management
+* Add client side parameter checking via Pydantic or TypedDict interfaces
 
 ### Features
 * Add `partition_async` for a non blocking alternative to `partition`
 
 ### Fixes
+* Address some asyncio based errors in pdf splitting logic