Add and edit rOpenGov general description (about)

Add rOpenGov general description (found at website [about](http://ropengov.github.io/about/) page and an email sent by @antagomir) to a new subfolder `ropengov-guidelines`.

Original `about.html` file was converted to a markdown file `about.md` in c0c09cf67d656ad23c9de15c9259f98f2c74bffc using pandoc. I then edited the content in this file, use GitHub's new render functionality to see [what has changed](https://github.com/jlehtoma/guidelines-docs/commit/show/a63f59096aa67cf800155244afc5bf69af6c6e7d/141c12a#diff-141c12a783887f3f5d72efae9694b8ae). See also [the actual source diff](https://github.com/jlehtoma/guidelines-docs/commit/a63f59096aa67cf800155244afc5bf69af6c6e7d) for additional line-specific comments.

In case the above is not complicated enough, `about.md` can be used as a drop-in replacement for `about.html` in [ropengov.github.io-website](https://github.com/rOpenGov/ropengov.github.io) with minor modifications. I've created [a new branch](https://github.com/rOpenGov/ropengov.github.io/tree/md-about) that demonstrates this.