Understanding content now set up in WCAG 2.1 repository

One clearly missing thing from WCAG 2.1 has been the Understanding 
content for proposed Success Criteria, to help our own and public review 
as well as eventually to support implementation. I've added support for 
this to the repository now and added basic documentation to the readme 
file https://github.com/w3c/wcag21/blob/master/README.md.

In short, there is a new folder "understanding", with a subfolder "21", 
and a file there for each success criterion proposal. The files 
currently have a template to get you started in filling in the content. 
SC managers will need to populate these files; in most cases starter 
content was provided with the issue proposals and you can refine as 
discussion progresses from there. Work on these in the same issue branch 
you're using for the SC itself.

The guidelines page now has automatic links to the Understanding 
document for each success criterion. The styling on that needs work but 
you can see it in the four SC that are currently in the draft:

  * https://rawgit.com/w3c/wcag21/master/guidelines/index.html#resize-content
  * https://rawgit.com/w3c/wcag21/master/guidelines/index.html#graphics-contrast
  * https://rawgit.com/w3c/wcag21/master/guidelines/index.html#interruptions-minimum
  * https://rawgit.com/w3c/wcag21/master/guidelines/index.html#accidental-activation

For those four SC, I filled out the Understanding pages with the content 
from the template so you can see how it looks all put together. Refer to 
those examples if need as you put content in the other files. Let the 
chairs or me know if you need help populating these files for your SC - 
but Understanding has a lot of content, so please help us spread the 
load if you can.

I have not included Understanding content from WCAG 2.0 in this, we will 
sort that out later when we decide whether or which WCAG 2.0 SC might be 
changed etc. I have also not set up Techniques yet - there are more 
complexities to that so I wanted to get people started on Understanding 
first.

Michael