message: document windows build
diff: # sovrin-client-rust
  +
  +# Windows build
  +
  +- Get binary dependencies (libamcl*, openssl, libsodium, libzmq, sqlite3).
  +- Put all *.{lib,dll} into one directory and headers into include/ subdirectory.
  +- Point path to this directory using environment variables:
  + - set SOVRIN_PREBUILT_DEPS_DIR=C:\BIN\x64
  + - set SODIUM_STATIC=y
  + - set SODIUM_LIB_DIR=C:\BIN\x64
  + - set OPENSSL_INCLUDE_DIR=C:\BIN\x64\include
  + - set OPENSSL_LIB_DIR=C:\BIN\x64
  + - set LIBZMQ_LIB_DIR=C:\BIN\x64
  + - set LIBZMQ_INCLUDE_DIR=C:\BIN\x64\include
  +- open MSVS development console
  +- execute "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvars64.bat"
  +- change dir to sovrin-client and run
  + - "cargo test" or
  + - cargo test --release --target x86_64-pc-windows-msvc
  