readme: editted install/build instructions for clarity
readme: editted install/build instructions for clarity

* some re-org of sections to make things more concise
* dedicated section for installing from packages vs building from source
* for Windows, instruct to run the top-level makefile targets,
  because I don't see any reason not to -- advanced users
  will look into the makefile and find the cmake command there
  if they need to.
* for Windows, instruct to install *-toolchain meta package
  which includes expat
* added some context info here and there to aid -- I tried
  to clarify things that confused me, e.g. Windows build is
  cross-compilation to be precise; motivation for targets
  that are meant for generating portable binaries as opposed
  to binaries optimized to host CPU.

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