Clean up 'contributing' section of readme

[gregjd]

The Contributing section of the readme is perhaps a bit overwhelming for someone who's a first-timer looking to see how to work on the website.

Thing big issue that I see is that it presents people with three choices on how to get the site running locally, which might be a bit overwhelming / confusing to someone who might not even understand what all of the options mean.

I'm not 100% on the best way to address this. Personally, I've never used Docker for working on codefordc.org, instead just running Jekyll locally. That has seemed to me to be the easiest way to do things. Is it genuinely easier if you use Docker? (If we do show multiple options, I'd make it very clear that one is the recommended option.)

[stvnrlly]

Looking at the instructions again (I think I wrote most of them), I think they're probably both too much and unhelpful.

The reason that those instructions are there is the number of people who wanted to make a change to the site and had a rough time just installing Jekyll. But I think trying to teach them package management in this README is probably a mistake.

Looking at the 18F site's README, it hits a couple options but still reads very cleanly. That might be a good model. For those who are interested in more, there's a separate CONTRIBUTING.md that goes in depth.