Add shared folder comment to README

I ran into an issue with GIT inside the VM, in which GIT thought every single file had changed file permissions settings.  The problem was possibly in part because the repos were originally in the VM, and moved to the shared folder, but it seems to be an issue with the execute bit not being available to set, or being masked out when the file is accessed through the shared folder.  The workaround as noted in the VM is to change a git config setting to tell git to ignore file mode modifications.