Disable shared sentencepiece libraries in installation instructions

[LoicGrobol]

The current build instructions for sentencepiece are not suitable if you don't have admin rights to install shared libraries. A more robust approach (as taken by https://github.com/google/sentencepiece/blob/master/python/build_bundled.sh) is to disable building shared libraries and statically link them in spm_train. If you want I can change the installation instructions to reflect this.

[prajdabre]

Please feel free to make the changes and send a PR!