Issue #421 -"Error in Windows Build Instructions"

[Forestidia86]

Issue#421
There is technically no error since if you download ldc2-1.26.0-windows-x64.7z as said in the build notes there is only one lib folder.
Dullstar probably downloaded the multilib version to which there is the installer as well, so it is nevertherless right to add the instructions for multilib as well.

(The build notes state explicitly: "Download the D compiler LDC, version 1.21.0 or newer;
choose the download that ends in "-windows-x64.7z":"
Nevertheless multilib should become standard since that's what the installer installs, Win 7 and Wine have only at the moment the known problem with LLVM. lib64 is as well the name for the 64-bit folder with dmd as well, so this would benefit from a rename as well.)