Update crossgen.md
Update crossgen.md

Add instructions on how to use the /r switch. Update instructions to use /p instead of /Platform_Assemblies_Paths (shorter version)
krk/coreclr,cshung/coreclr,krk/coreclr,cshung/coreclr,poizan42/coreclr,wtgodbe/coreclr,poizan42/coreclr,poizan42/coreclr,krk/coreclr,poizan42/coreclr,wtgodbe/coreclr,poizan42/coreclr,cshung/coreclr,cshung/coreclr,krk/coreclr,poizan42/coreclr,wtgodbe/coreclr,wtgodbe/coreclr,cshung/coreclr,krk/coreclr,cshung/coreclr,wtgodbe/coreclr,wtgodbe/coreclr,krk/coreclr