Update readme with link to foss4g workshop material

[Robinlovelace]

Heads-up @rsbivand this is now what I get after the changes above:

 initGRASS()
No gisBase set. Trying to detect from the GRASS_INSTALLATION environment variable
No GRASS_INSTALLATION environment variable found
Trying to find it with the system command:
grass --config path # if this fails set gisBase manually
Error in initGRASS() : 
  A GISRC file already exists; to override, set override=TRUE

Restarting R session...

> library(rgrass)
> initGRASS()
Error in initGRASS() : 
  A GRASS location is already in use; to override, set override=TRUE
> initGRASS(override = TRUE)
No gisBase set. Trying to detect from the GRASS_INSTALLATION environment variable
No GRASS_INSTALLATION environment variable found
Trying to find it with the system command:
grass --config path # if this fails set gisBase manually
gisdbase    /tmp/RtmpL1izBs 
location    file5f2d552956883 
mapset      file5f2d5215b1121 
rows        1 
columns     1 
north       1 
south       0 
west        0 
east        1 
nsres       1 
ewres       1 
Warning message:
In execGRASS("g.region", flags = c("g", "3"), intern = TRUE, ignore.stderr = ignore.stderr) :
  The command:
g.region -g -3
produced at least one warning during execution:
WARNING: <PROJ_INFO> file not found for location <file5f2d552956883>
WARNING: <PROJ_UNITS> file not found for location <file5f2d552956883>

Bad news: new error message without override = TRUE.

Good news: seems to work with override = TRUE.

This probably needs iteration if it's viable at all but think 'make it easy if possible but encourage people to manually set their gisBase location' is reasonable. Heads-up also @veroandreo your comments/review on this also v. welcome (and other GRASS developer people).