File versioning and readMe documentation

Added text recommending file versioning and creation of readMe documentation for data manipulations.
Merge pull request #57 from wrightaprilm/master

Merging @chenghlee and @aaronreba python panda lessons
Merge pull request #33 from aNurnberger/patch-2

File versioning and readMe documentation
Update sql.md file
Merge pull request #60 from jrherr/master

Update sql.md file
Upload ggplot2.Rmd exercise
Updated link for SQL data and moved Ethan's file

Updated link and moved Ethan’s file from SWC repo to local DC repo
Merge pull request #61 from jrherr/master

Added ggplot2.Rmd exercise
Add attribution for SQL lessons
Add blank lines after markdown headings

This makes the documents easier to read and allows text editors
to correctly handle the text under the headings.
Add license file

Adds a license file based on the SWC license file, which is CC-BY
for material plus MIT for associated software.

Fixes #22.
Add attribution for shell lectures

Attribution determined by looking at the git logs from the SWC
bc repositority for the shell folder.
Merge pull request #63 from ethanwhite/add-attribution

Add attribution for SQL & shell lessons
Merge pull request #64 from ethanwhite/heading-spaces

Add blank lines after markdown headings
Use Female/Male and other changes to tidy-data/00-intro
Add untidy data left behind for exercise in tidy-data/01-tidy
Merge pull request #65 from ethanwhite/license

Add license file.
Add lesson on how to tidy in tidy-data/01-tidy
Link to second tidy-data lesson from first, now that it exists
Add supplemental material to lesson on data tidying