Update CHANGELOG.rst (#694)

Author: MatthiasSchmidtblaicherQC

CHANGELOG.rst

@@ -7,7 +7,7 @@
 Changelog
 =========
 
-2.6.0 - UNRELEASED
+2.6.0 - 2023-09-05
 ------------------
 
 **New features**
@@ -26,6 +26,7 @@ Changelog
 - When computing the covariance matrix, check for ill-conditionedness for all types of input. Furthermore, do it in a more efficient way.
 - Pin `tabmat<4.0.0` (the new release will bring breaking changes).
 
+
 2.5.2 - 2023-06-02
 ------------------