Instruction for chisel3 users

[hossein1387]

I added instructions for those who are using chisel3. With the current setup, the sbt publish command results in the following error:

check] ValExec_fNFromRecFN.scala:44: Chisel3 compatibility: io's should be wrapped in IO() in class hardfloat.ValExec_fNFromRecFN$$anon$1

My instructions fixes the issue.

[edcote]

I am not convinced that this approach works on scala 2.12, have you tried? I can compile for 2.11.11, but 2.12.4 appears broken.

[edcote]

See a proposed fix: https://github.com/ucb-bar/berkeley-hardfloat/pull/20