Improve headings in README

Overall, the README of the Redcarpet gem is good. But I've been bothered
about the headers for a while. The way they are written signals a couple
of things that I think the project should stay away from:

Stupidifying the user
----------------------------
Meaning can be delivered without overusing words like "like" and
"totally".

Hinting at century-old gender stereotypes
-----------------------------
Thousands of developers of any gender and background use this gem. Let's
keep references to family values from the 50's out of our language. (I'm
referring to "Darling, I packed you a couple renderers for lunch").

Calling things "boring"
----------------------------
Yeah, licensing might be "boring" to some. But it's at core of every
open source project, whether you like it or not.

In this commit, I've made an attempt at improving some headers. Most of
them were introduced in 427146c4b6e.