README Page for r-spatial

[nickbearman]

Hi,
Given our recent recognition as as OSGeo Community Project, I thought it might be useful if the R-Spatial GitHub Organisation has a README.md file included. This allows us to give a short description at the top of github.com/r-spatial and will allow us to pin (up to) six repos on that page. Given that GitHub is our primary home I think this might be a good idea. More details at https://docs.github.com/en/organizations/collaborating-with-groups-in-organizations/customizing-your-organizations-profile.

Additionally, it will give us some space to address the requirements to be an OSGeo Community Project (see #44).