Add doc to switch between different JDK versions

[shireeshaBongarala]

#111

[aahlenst]

In my experience, export JAVA_HOME=$(/usr/libexec/java_home -v 13) is sufficient. Alterations of $PATH rather hurt than help on macOS. /usr/bin/java is the default java on macOS and provides the magic that makes JAVA_HOME work. If you prepend the output of export JAVA_HOME=$(/usr/libexec/java_home -v 13) to $PATH, you break all the magic.

[shireeshaBongarala]

@aahlenst Thanks for the comment. I fixed the issue in my local $PATH to remove java paths other than /usr/bin/java. Now, just updating the JAVA_HOME env var is sufficient.
I updated the PR.