Update install documentation especially for Mac

[bryantaustin13]

From Zulip:
Last week I was having a hard time setting up the VSCode extension on a Mac. In my environment I use asdf to manage which versions of s/w packages I have, including Java. It wasn't obvious how to setup my environment to get VSCode to find my asdf version of Java. I would see errors like this when running the CQL -> Execute:

Starting client failed
Launching server using command /undefined/bin/java failed.

So I could tell it wasn't finding Java so I started trying to set JAVA_HOME, JDK_HOME, etc. but couldn't figure out the "secret sauce" until I got some help. (I suppose this isn't really a problem with the CQL Extension but it turned out to be a barrier for me)

It turned out that I had to add this to my ~/.zshrc (similar asdf scripts exist for other shells)

~/.asdf/plugins/java/set-java-home.zsh