Add documentation for building/installing on windows/macosx

[sydp]

To build on osx, I needed to install snappy using brew then run

CPPFLAGS="-I/opt/homebrew/include -L/opt/homebrew/lib" pip install python-snappy

[sebbu2]

To build on windows, i needed to install snappy via vcpkg (which i installed via scoop) then run

set "CL=-Ic:\Users\sebbu\scoop\persist\vcpkg\installed\x64-windows\include"
set "LINK=/LIBPATH:c:\Users\sebbu\scoop\persist\vcpkg\installed\x64-windows\lib"

Then I could install python-snappy or dfindexeddb[plugins]

If the dependencies used cmake,nit would have been
set "CMAKE_TOOLCHAIN_FILE=C:/Users/sebbu/scoop/apps/vcpkg/current/scripts/buildsystems/vcpkg.cmake"