message: update build instruction
diff: 1. Install [rustup](https://www.rustup.rs) toolchain manager.
  2. Install toolchains using command
  - > rustup update aarch64-apple-ios
  - > rustup update armv7-apple-ios
  - > rustup update armv7s-apple-ios
  - > rustup update i386-apple-ios
  - > rustup update x86_64-apple-darwin
  - > rustup update x86_64-apple-ios
  -
  -3. Edit script build-libsovrin-core-ios.sh: set the following variables to fit your environment:
  + > rustup target add aarch64-apple-ios armv7-apple-ios armv7s-apple-ios i386-apple-ios x86_64-apple-ios
  +
  +3. Install cargo-lipo
  + > cargo install cargo-lipo
  +
  +4. Install required native libraries and utilities
  + > brew install libsodium
  + > brew install zeromq
  + > brew install cmake
  +
  +5. Edit script build-libsovrin-core-ios.sh: set the following variables to fit your environment:
  > export OPENSSL_DIR=/usr/local/Cellar/openssl/1.0.2k
  > export EVERNYM_REPO_KEY=~/Documents/EvernymRepo
  > export LIBSOVRIN_POD_VERSION=0.0.1
  EVERNYM_REPO_KEY - path to file with private key to be authorized on deb server
  LIBSOVRIN_POD_VERSION - version of livsovrin-core pod to be built
  -4. Run the script. Validate the output that all goes well.
  +6. Run the script. Validate the output that all goes well.
  -5. cd Podspec
  -6. Create directory with name defined in LIBSOVRIN_POD_VERSION
  +7. cd Podspec
  +
  +8. Create directory with name defined in LIBSOVRIN_POD_VERSION
  mkdir LIBSOVRIN_POD_VERSION
  -7. copy libsovrin-core.podspec.json to that new directory from some previous version
  -8. edit this json -> change version field to LIBSOVRIN_POD_VERSION
  -9. add new directory and file inside to git repository.
  -10. commit to master branch
  -11. for all projects which using libsovrin-core do not forget to make
  +
  +9. copy libsovrin-core.podspec.json to that new directory from some previous version
  +
  +10. edit this json -> change version field to LIBSOVRIN_POD_VERSION
  +
  +11. add new directory and file inside to git repository.
  +
  +12. commit to master branch
  +
  +13. for all projects which using libsovrin-core do not forget to make
  > pod repo update
  > pod install
  