Release notes for 0.9.1 (#1964)

Summary:
0.9.1 is the same as 0.9.0 apart from a version bump to linear_operator that was required since the pinned 0.5.0 version had a bug.

Pull Request resolved: #1964

Reviewed By: saitcakmak

Differential Revision: D47968240

Pulled By: Balandat

fbshipit-source-id: 0fb99a1b35b01a48dc3d6e27599689724f2145d5

Author: Balandat

CHANGELOG.md

@@ -2,7 +2,12 @@
 
 The release log for BoTorch.
 
-## [0.9.0] - July 31, 2023
+## [0.9.1] - Aug 1, 2023
+
+* Require linear_operator == 0.5.1 (#1963).
+
+
+## [0.9.0] - Aug 1, 2023
 
 #### Compatibility
 * Require Python >= 3.9.0 (#1924).