update changelog for alpha release

Author: puredanger

README.md

@@ -64,6 +64,8 @@ Copyright © Rich Hickey and contributors
 
 ## Changelog
 
+* Release 1.10.870-alpha2 on 2026-04-06
+  * [ASYNC-275](https://clojure.atlassian.net/browse/ASYNC-275) (CLJ) Fixed flow/futurize to retain original behavior of propagating exceptions
 * Release 1.10.864-alpha1 on 2026-03-19
   * Alpha release of core.async.flow - all APIs subject to change
   * [ASYNC-274](https://clojure.atlassian.net/browse/ASYNC-274) (CLJ) Fixed flow/futurize stopped returning a future on Executor change