Update shell_tools_101.rst

Include df, du, find, and ls in the list of commands for working with your system.
Update meta/contributions.rst

Added self to list of contributors.
Update meta/contributions.rst

Added my email (jdm@dea-mattson.com).
Merge pull request #1 from jordandm/patch-1

Update shell_tools_101.rst commands included to include cut, df, du, find, ls, and sort
Update shell_tools_101.rst

Fix formatting for sort section.
Update shell_tools_101.rst

Fix sort formatting.
Update shell_tools_101.rst
Update shell_tools_101.rst

Fix command heading formatting.
Merge pull request #53 from jordandm/master

Added self to contributors; various fixes.