Added CMake build 'badge'

Author: cwoffenden

README.md

@@ -1,5 +1,7 @@
 # Optimisation Task #1
 
+[![CMake workflow](https://github.com/numfum/task-opt1/actions/workflows/cmake-desktop.yml/badge.svg)](https://github.com/numfum/task-opt1/actions/workflows/cmake-desktop.yml)
+
 This is a programming task given to candidates interviewing at [Numfum GmbH](//www.numfum.com/jobs_en/) to complete from home, taking as long as they need. The code is a snippet from [Basis Universal](//github.com/BinomialLLC/basis_universal) (copyright 2019-2021 Binomial LLC, released under an Apache 2.0 license). The [original code](//github.com/BinomialLLC/basis_universal/blob/77b7df8e5df3532a42ef3c76de0c14cc005d0f65/transcoder/basisu_transcoder.cpp#L1178-L1253) is extracted from the ETC1S to DXT transcoder and turned into this standalone sample.
 
 ### The Task