www/chromium: Workaround description for the hanging tabs problem

Thanks to the solution suggested by Dmitri Goutnik, I am amending
pkg-message with the workaround for the hanging tabs problem.

Putting ~/.cache/chromium on memory-fs solved the problem for me.
Now chromium doesn't hang even on a heavily loaded machine.

PR: 226793
Reported by: Dmitri Goutnik <dg@syrec.org>
Approved by: Carlos J. Puga Medina <cpm@freebsd.org> (maintainer)