Add `opam update` to install instructions
Add `opam update` to install instructions

There's never enough `opam update`s.
facebook/reason,yunxing/reason,facebook/reason,yunxing/reason,cristianoc/reason,ak3n/reason,ak3n/reason,ak3n/reason,let-def/reason,yunxing/reason,chenglou/reason,ak3n/reason,let-def/reason,cristianoc/reason,cristianoc/reason,chenglou/reason,facebook/reason,yunxing/reason,chenglou/reason,let-def/reason