Added Spark PCA result wrangling commands
Correction to dataset size.

The BigQuery table is 3.34TB uncompressed but the actual uncompressed files on Google Cloud Storage are smaller.

gsutil du -sh gs://genomics-public-data/1000-genomes/vcf
1.12 TB     gs://genomics-public-data/1000-genomes/vcf
Add call to GoogleGenomics::authenticate
Update AllModalitiesDemo.md

Missed a spot for commit https://github.com/googlegenomics/codelabs/commit/0383d60be98771c2bbfe4b9705878400b535a045
Update AllModalitiesDemo.Rmd

Missed a spot for commit https://github.com/googlegenomics/codelabs/commit/0383d60be98771c2bbfe4b9705878400b535a045
Add instructions for contributions.
Merge branch 'master' of github.com:googlegenomics/codelabs