Update 06_html.md

Added an outline for introducing basic accessibility concepts related to HTML and CSS.
Update 06_html.md

fixing headings
Add missing newline
Fix image path
Create 06_html_a11y.md

Proposing HTML/CSS accessibility intro outline.
Adding Accessibility Section

Added the Accessibility Section. Feel free to add more :).
added IRC instructions to introduction.md
incorrect week label
Clean up mvc lesson and add a link to the wiki
Tweak model description
Nitpicky grammar fix
Adding DOM manipulation details to the lessons
Revert "Adding DOM manipulation details to the lessons"

This reverts commit 2fe2ce3fc199941f52909b359774ff9b4d15cf42.
Nitpicky grammar fix

Plus some git cleanup

Nitpicky grammar fix

Adding DOM manipulation details to the lessons

Revert "Adding DOM manipulation details to the lessons"

This reverts commit 2fe2ce3fc199941f52909b359774ff9b4d15cf42.
Merge branch 'master' of https://github.com/jlembeck/curriculum
Merge pull request #26 from jlembeck/master

Nitpicky grammar fix
Merge pull request #25 from RobinClowers/mvc-updates

Clean up mvc lesson and add a link to the wiki
cleaned up the irc instructions at the end of introduction.md
Merge pull request #28 from Ada-Developers-Academy/irc_instructions_take_2

cleaned up the irc instructions at the end of introduction.md
Merge pull request #23 from RobinClowers/rails-fix-image

Fix rails stack image