Update README

[fboehm]

1. remove text about rstudio session in the cloud.
2. add text and hyperlinks about multivariate, one-QTL scans

[fboehm]

Also need to fix equation renderings, especially the bivariate model equation.

[fboehm]

equation rendering is now fixed, but the other components need updates.

• remove text about R session in the cloud
• add text about multivariate, one-QTL scans
• add hyperlinks to new vignette