[SIMD] add Changelog entry

CHANGELOG.md

 # Master (will become release 2.9)
 
+- Solvers are more robust if used with multiple right-hand sides and one lane starts with the exact solution.
+
 - Added a function to write nested matrices as SVG objects: `writeSVGMatrix(...)`
 
 - `MultiTypeBlockVector` uses now `std::common_type` of the entries for the `field_type`. The old `double`