message: add openssl-sys workaround description
diff: - set static flag for libsodium build
  - set SODIUM_STATIC=y
  - change dir to sovrin-client and run cargo (you may want to add --release --target x86_64-pc-windows-msvc keys to cargo)
  +
  +## openssl-sys workaround
  +
  +When your windows build fails complaining on gdi32.lib you should edit
  +
  +```
  + ~/.cargo/registry/src/github.com-*/openssl-sys-*/build.rs
  +```
  +
  +and add
  +
  +```
  + println!("cargo:rustc-link-lib=dylib=gdi32");
  +```
  +
  +to the end of main() function.
  +
  +Then try to rebuild whole project.
  