[readme] instructions for linux pointing out the need for opam >= 1.2.2
[readme] instructions for linux pointing out the need for opam >= 1.2.2

ak3n/reason,facebook/reason,cristianoc/reason,ak3n/reason,let-def/reason,yunxing/reason,cristianoc/reason,yunxing/reason,yunxing/reason,chenglou/reason,ak3n/reason,facebook/reason,let-def/reason,cristianoc/reason,yunxing/reason,let-def/reason,ak3n/reason,chenglou/reason,chenglou/reason,facebook/reason