Use h3 headings for build sections in README

[eklitzke]

I think the sections about different compiler options should be sub-sections of the "Building" section. This change does that -- the LTO and profile-guided optimization sections are now "h3" sections.

You can see how this renders on GitHub at https://github.com/eklitzke/cpython/tree/docs

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[methane]

Thank you for suggestion.
How about adding TOC before "Build Instructions"?

.. contents::

(ref github/markup#391)

[methane]

Please use ^^^^^^^ as subsubsection header.
https://docs.python.org/devguide/documenting.html#sections

[eklitzke]

Add TOC, and used ^^^^ for delimiting the subsections.

[methane]

LGTM.
Please sign CLA and wait it applied.

[codecov]

Codecov Report

Merging #64 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #64   +/-   ##
=======================================
  Coverage   82.37%   82.37%           
=======================================
  Files        1427     1427           
  Lines      350948   350948           
=======================================
  Hits       289091   289091           
  Misses      61857    61857

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[methane]

@eklitzke Could you sign the CLA?
step is described by #64 (comment)