CMake: Add package version file for installation

[robamu]

Hi,

CMake provides the option to write a package version file. The documentation for this can be found here.

I think this is really useful if FetchContent and find_package are used to find an installed ETL library. From what I have seen, I was not able to use the version feature, for example when providing a major version requirement like this:

find_package(etl 20 QUIET)

Could be that I misused the feature or did something wrong so correct me if I am wrong. If I omit the version number, CMake is able to find the installed ETL library without issues.

Also I think it would be nice if installation instructions are provided for CMake.

I followed the following steps to install a specific version of ETL

git clone https://github.com/ETLCPP/etl
cd etl
mkdir build && cd build
git checkout <someVersion>
cmake .. && cmake --install .

[jwellbelove]

I will take a look, but I'm no expert in cmake.

[robamu]

I hope this is not already something you have been working on, but I opened a PR which introduces a little bit of version handling to the CMake file:
#522

[jwellbelove]

Added 20.27.2