Add prereq/requirement to the Manual Install section/homepage

[naxels]

This might sound very obvious and redundant given that bbin is part of the babashka github repo,

However I found that for both the homepage, https://radsmith.com/bbin
as well as the Manual installation md, https://github.com/babashka/bbin/blob/main/docs/installation.md

There is no mention of the prerequisite to bbin: Babashka itself.

The Brew install script has a dependency on Babashka, but for the folks that are new to this and
for some reason need a manual install, would end up with a not working script, thinking they followed
the steps correctly.

I wanted to create a PR for the installation.md file, but given multiple Manual 'paths' depending on the OS
don't know where to put it.

Thank you for making this tool :)