README: fix title levels

[Artoria2e5]

"Using thisthat" is under installation, so I moved these to level-2 titles. This should make it read more logical.

I don't really know how to rephrase the top so the biggest title is just one line.

[Artoria2e5]

Can we... figure out a way to describe a maximum common part of the code-style of this entire repository and apply it using clang-format? This will make reading and refactoring this thing easier in the long run.

What I have so far is:

---
BasedOnStyle: WebKit
# This is usually bigger
ColumnLimit: '120'
IndentPPDirectives: BeforeHash
IndentWidth: '4'
TabWidth: '4'
UseTab: ForIndentation
SpaceAfterCStyleCast: true

# DerivePointerAlignment: true
PointerAlignmentStyle: Left

# This is not uniformly done
SpacesInParentheses: true
...

Note that IndentPPDirectives DOES detect include guards, but not in the way they current look like. You must make it look like #ifndef XXX followed by #define XXX immediately, and have it cover the entire file with no #else. In the repo the define is currently at the end, so the guards miss the detection.

[RJVB]

I can't say I'm very motivated to spend any significant amount of time on that. Not that I don't see the point but I wasn't even planning on doing more than corrective maintenance on the tool as it currently is, so making the code more readable or refactoring it aren't on my agenda. Of course you can reformat as much as you want, and I'll see whether or not I have any issues with the result when you're done. Sorry...