Document OS X incompatibilities

[octotep]

It is my ultimate goal for menugen to run on OS X, however a few oddities prevent that.

1. bash is ancient. menugen requires at least version 4.0 for hash tables in bash. It also currently requires bash 4.2+ because of its declare -g statements, but they might be redundant. I will investigate that.
2. md5sum doesn't exist. You can install it via Homebrew with brew install md5sha1sum
3. Some sed commands might not work due to the difference between BSD utils and GNU utils. This might actually be fixable if reported. Alternatively, one could just install GNU sed.

Also rofi doesn't support OS X - only the CLI interface with fzf works.

[flamwenco]

https://github.com/uluyol/xmenu