Update intro_to_shell.md

Added link to slides in overview
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Update intro_to_shell.md
Update intro_to_shell.md

added note to Soft links
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edited directory exercise so that work directory is not deleted
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changed pipe and redirect example to use /usr/bin instead of /home so as not to cause big wait.
Update intro_to_shell.md

Fixed issue with pipe example coming too early, and moved file creation to start.
Merge pull request #3 from tcouch/patch-1

Aristotle-shell-update