add introduction to p@w3c home page

[dauwhe]

I thought w3.org/publishing could use an introduction, saying a tiny bit about why we're here. I've written a few paragraphs, but consider this just the start of a discussion.

[iherman]

I like this. Before merge, I would like to have Ralph's and Karen's reaction, too.

I will send them a mail referring to:

https://tinyurl.com/ycxpwawc

a.k.a.

http://htmlpreview.github.io/?https://raw.githubusercontent.com/w3c/publishing/3f92a0218a58f0c023baf02640e148809446ec74/index.html

(I do cc this to @swickr , too, but Karen, afaik, does not use github...)

[iherman]

Thanks @dauwhe !