Updating README and LAYOUT to reflect addition of CONTRIBUTING

what it's for and how to edit it for each lesson.

A generic CONTRIBUTING file

Minor fixes to license file.

1.  Removed YAML header.
2.  Changed link to refer to `LICENSE.html` instead of `LICENSE.md`.

Leaving the reference to the lesson-template repo alone - lesson authors will be expected to edit this (and instructions to do that will go in the docs in the gh-pages branch).

Validator should fail when FIXME present

Add validator target to makefile

Update validator to work with revised LICENSE.md.

1.  Remove copyright information about code of conduct (which isn't in this repo).
2.  Change ownership of trademark to NumFOCUS.
3.  Use uniform link syntax.

Removing FIXME from instructors.md

First should come the common workflow tasks, then commands for internal/debugging use.

Our authors use this label to mark unfinished/work in progress lessons.

Sync gh-pages with newest core changes

Fix validation of motivation.md

Revert changes when validating glossary

Skip mailto links in link validation

Removing unused files. Thanks @gvwilson, it looks good.

Merging CSS from styles repository

Merge branch 'update-template-no-css' of github.com:fmichonneau/lesson-template into update-template-no-css