Amend "Developing Locally" portion of README with lnd build flag instructions

[valentinewallace]

To be able to pass the integration tests, you'll need to build LND commit 51d43be1f369c7ec5ae8bc3bb282a45c6a0776ea with experimental build flag as in this line. The README should be edited to reflect this.

[blockchainsolutionshi]

So I rebuilt LND with the experimental tag and it still throws the same error

@blockchainsolutionshi did you also copy over the binary to ./assets/bin?

For OSX:
cp $GOPATH/bin/lnd ./assets/bin/darwin

[blockchainsolutionshi]

That worked thank you