Improve automatic adjustment of "README.md"

[elfring]

Some commands are used to update the file "README.md".

• How do you think about to work with another template file? Would it eventually be better and safer to reuse the CMake functionality "configure_file"?
• If you insist on in-place modification here, I suggest to drop the variable "MODIFIED_FILE_CONTENT" because of the detail that a direct use of the variable "FILE_CONTENT" would be sufficient there, wouldn't it?

[naelstrof]

Instead I decided on a shell script, giving me more flexibility and readability on how the README.md is updated.

I also made it a custom target, so that it's only executed when asked and properly keeps track of dependencies: https://github.com/naelstrof/maim/blob/master/CMakeLists.txt#L65

I believe this is pretty safe, and clean. Feel free to re-open if you feel otherwise.

[elfring]

Can it be that this shell script is not as portable and efficient as an approach where you would stick to the CMake programming language in the corresponding build scripts?