Add project vision to Readme

This is a rough first pass at memorializing some of the project vision discussed in https://github.com/jekyll/jekyll/issues/1929. 

Often times, as projects grow and gain popularity, it's helpful to have a short description of the project's vision and design philosophy to serve as a lense through which to evaluate proposed features and approaches.

I also tried to update the project description a bit, to better reflect how its often being used today. FWIW, it [looks like the blurb hasn't been updated in five years](https://github.com/jekyll/jekyll/commit/5a1d736242f8382e0c97b928b7eeaeb674676321)

What I was trying to reflect:

> * “Jekyll is a simple, blog aware, static site generator.”
> * Blog aware, not blog centric 
> * File-system based CMS
> * Originally called “autoblog”
> * Despite start, big use case today is technical documentation
> * Not Rails style conventions over configurations
> * Don’t try to outsmart the users
> * Jekyll should do what I tell it to do, no more, no less
> * The three-little bears of CMSs
> * Git should not be a requirement to using Jekyll
> * Marketing for open source projects, people, organizations
> * Make it easier to install (Installer Packages with Ruby/RubyGems & gem dependencies installed)

Again, a very rough first pass, but thought it was at least worth getting the conversation started if there was interest in writing down some of the unsaid assumptions that surround the project's ethos.