Add dependency boto3 to readme.md (#1741)
Summary:
There is an import error for `boto3` for a new install. This dependency has been added to https://github.com/pytorch/benchmark/blob/2b9de6b084b8da25bea45faf74ae267c374eb9ee/scripts/install_conda.sh#L11
and other places but missed in readme.
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1741
Reviewed By: xuzhao9
Differential Revision: D46902604
Pulled By: FindHao
fbshipit-source-id: b0d425d3e0c3b8c90ba9a12898c0e9e70d60a681
