Merging core into gh-pages

Adding contributor covenant

Fixes to lesson layout instructions

there. >_< )

Changed the reference to the lesson's README file -- it's not "this file" any more (this file is LAYOUT.md). 

Also removed the ">"s from the beginning of the block of text to be included in the README, to enable copying-and-pasting (and because the ~~~ should offset the quote by itself. I hope.

Load bootstrap from CDN

Add documentation for writing lessons in R Markdown.

Revert "Fix link to git pro book"

Fix link to git pro book

Relates to #172

Add more lables to to setup labels script

Add labels for issues and PRs

This labels was suggested by @abought and @willingc.

Do not render CONTRIBUTING.html. Ignore it if it is built.

Fix link to Pro Git chapter on contributing to a project.

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.