Added help for the SharedDirectoryInstaller according to Jesse Glick's comment
URL:
https://github.com/jenkinsci/extra-tool-installers-plugin/commit/70ad1fa728c2c353d4f6baf9836ba86d40a43ebe#commitcomment-3687460

Signed-off-by: Oleg Nenashev <nenashev@synopsys.com>