take out mention in README that R must be on PATH; no longer needed
take out mention in README that R must be on PATH; no longer needed

This reverts commit 7417db49ca1ec7bb265f19c7928bf37fc6058a6e.

gdtm86/IRkernel,Phobia0ptik/IRkernel,elaOnMars/IRkernel,ibm-et/IRkernel,ibm-et/IRkernel,ChinaQuants/IRkernel