[R] Update README.md
[R] Update README.md

Update for feather being on CRAN and having binaries. Addresses https://github.com/wesm/feather/issues/153.

Author: Avraham Adler <aadler@users.noreply.github.com>

Closes #172 from aadler/patch-1 and squashes the following commits:

579cebb [Avraham Adler] Update README.md

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