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
Explain tidying data lesson in its README
Internal links in the data tidying lesson
Enrich README for tidy data lesson
Move SQL reference material to end of lesson

The SQL reference material added in 56221b3c7bb is useful, but
shouldn't be a focus when delivering this lesson in workshops
since we are assuming complete novices and presenting this level
of detail up front would be too much.

This change moves the material to the end of the lesson and
provides hyperlinks to it in the main material to highlight its
existence and make it easy to jump to when viewing the material
online.
Merge pull request #67 from ethanwhite/move-sql-ref-mat

Move SQL reference material to end of lesson.
Polish tidy-data lesson
Fix typo that broke a link
Responding to @ethanwhite comments on pull request for tidy data lesson