re-word README re: expiring credentials
re-word README re: expiring credentials

SecurityInsanity/rusoto,mthjones/rusoto,rusoto/rusoto,mattico/rusoto,noggleboggle/rusoto,mattico/rusoto,DualSpark/rust-aws,matthewkmayer/rust-aws,ford-prefect/rusoto,SecurityInsanity/rusoto,DualSpark/rust-aws,rusoto/rusoto,matthewkmayer/rust-aws,cmsd2/rusoto,DualSpark/rust-aws,ford-prefect/rusoto,mthjones/rusoto,matthewkmayer/rust-aws,rusoto/rusoto,DualSpark/rusoto,DualSpark/rusoto,mattico/rusoto,indiv0/rusoto,DualSpark/rusoto