add cancer_expression.rds to Setup instructions

[smcclatchy]

The principal components analysis and statistical models episodes load cancer expression and HCMV data which is not included in setup instructions. Add data download instructions for cancer_expression.rds and hcmv.rda

Please download these two data files into your Desktop/inference/data directory:

breast cancer gene expression:  https://github.com/smcclatchy/inference-for-high-dimensional-data/blob/gh-pages/data/cancer_expression.rds

HCMV: https://github.com/smcclatchy/inference-for-high-dimensional-data/blob/gh-pages/data/hcmv.rda