Document need for GO15VENDOREXPERIMENT=1.
Document need for GO15VENDOREXPERIMENT=1.

Fixes #425.

decred/dcrwallet,synrg-labs/btcwallet,Stash-Crypto/btcwallet,Roasbeef/btcwallet,decred/dcrwallet,synrg-labs/btcwallet,Roasbeef/btcwallet,jrick/dcrwallet,btcsuite/btcwallet,jrick/dcrwallet,jrick/btcwallet,Stash-Crypto/btcwallet,jrick/btcwallet,btcsuite/btcwallet