Adding a simple Python-as-calculator exercise.
Merge remote-tracking branch 'jiffyclub/python-calculator'

* jiffyclub/python-calculator:
  Adding a simple Python-as-calculator exercise.
shell/Readme.md: Fix 'swsc' -> 'swc' typo
Merge pull request #25 from wking/swsc-typo

shell/Readme.md: Fix 'swsc' -> 'swc' typo
Update shell/Readme.md

Fix typo and some missing words.
Merge pull request #27 from douglatornell/patch-1

Fix typo and add some missing words in shell/Readme.md
Update shell/Readme.md

Improve wording in `cp` file backup example.
Update shell/Readme.md

Fix markup to highlight `find` command.
Merge pull request #28 from douglatornell/patch-2

Update shell/Readme.md
Merge pull request #29 from douglatornell/patch-3

Update shell/Readme.md