README: clone without a GitHub account

[zaz]

Previously, the README instructions would require a GitHub account setup with SSH to clone the repository. The only advantage I can see of doing things this way is if you had permissions to push updates to vubiostat/Renv. Using https: allows anyone to clone this.

I also removed cd in favor of specifying the clone location at the end of the clone command: This has the advantage that it's one line instead of two, and it's more transparent (I believe more people are familiar with ~ than the fact that cd defaults to taking you to your home directory).