Add cmake file

[tplet]

No description provided.

[PaulStoffregen]

How is this used? This library is meant to be used from Arduino.

[tplet]

Arduino project can be compiled also from another IDE than Arduino. In my case, i use CLion from Jetbrains. Also, it's possible to compile arduino project from eclipse IDE or Netbeans.
In all these cases, a cmake file permit to save compilation configuration - and don't have to create again each time on each environment...

In my case, on CLion, i use your library as dependencies of project by including as git submodule.
Cmake helper look at each directory included and read CMakeList.txt file

More information here : https://cmake.org/