update README for v2.7

Author: dkarrasch

README.md

@@ -8,6 +8,9 @@ A Julia package for defining and working with linear maps, also known as linear
 transformations or linear operators acting on vectors. The only requirement for
 a LinearMap is that it can act on a vector (by multiplication) efficiently.
 
+## What's new in v2.7
+*   Speed-up of scaled `LinearMap`s by avoiding allocations
+
 ## What's new in v2.6
 *   New feature: "lazy" Kronecker product, Kronecker sums, and powers thereof
     for `LinearMap`s. `AbstractMatrix` objects are promoted to `LinearMap`s if