Update README for Go 1.11 and module builds.
Update README for Go 1.11 and module builds.

The building from source section has been simplified for the instructions to
perform a module-aware build.

For the dcrwallet 1.4 development cycle, Go 1.9 is no longer supported, and
module builds must be performed with Go 1.10 or Go 1.11 instead.

While here, remove all references to Windows MSIs, as none are provided.

jrick/btcwallet,jrick/btcwallet,decred/dcrwallet,jrick/dcrwallet,jrick/dcrwallet,decred/dcrwallet