README.md is outdated regarding shared libs

It seems https://github.com/DataDog/zstd/commit/af19ae6bd5af997c006a9e6dfcbf24a211d24e18 forgot to change the README:

> All C files from Zstd have been removed from the distribution; instead the shared library is used. This version has been tested against zStd library version 1.0.
