Add section on running the site locally using Jekyll to the README

I took a stab at writing this section. Feedback welcome, particularly on what the title of this section should be to maximize clarity.

It seems like the README is trying to keep the barrier low to entry, so I don't know if we'd want to caveat this (eg, this dev path is targeted at folks who have basic familiarity web development and git/GitHub processes) but here's a first cut since it actually took some guesswork to figure out this was the local dev flow on my end.

*(Ed. note: If I'm going to debate this on Twitter, I should be the change I want to see in the world.)*