Add optional section for using lld-link on Windows

README.md

@@ -221,13 +221,29 @@ settings for the installer are fine).
 
 5. Install Visual Studio Community 2017 (https://www.visualstudio.com/vs/community/). You MUST add "Visual C++" to the
 list of installed components. It is not on by default. Visual Studio 2017 MUST installed to the default location or mach.bat will not find it.
+
+##### [Optional] Install LLVM for faster link times
+
+You may experience much faster builds on Windows by following these steps. (Related Rust issue: https://github.com/rust-lang/rust/issues/37543)
+
+1. Download the latest version of LLVM (http://releases.llvm.org/).
+2. Run the installer and choose to add LLVM to the system PATH.
+3. Add the following to your Cargo config (Found at `%USERPROFILE%\.cargo\config`). You may need to change the triple to match your environment.
+
+```
+[target.x86_64-pc-windows-msvc]
+linker = "lld-link.exe"
+```
+
+##### Troubleshooting a Windows environment
+
 > If you encountered errors with the environment above, do the following for a workaround:
 > 1.  Download and install [Build Tools for Visual Studio 2017](https://www.visualstudio.com/thank-you-downloading-visual-studio/?sku=BuildTools&rel=15)
 > 2.  Install `python2.7 x86-x64` and `virtualenv`
 > 3.  Run `mach.bat build -d`.
 
 >If you have troubles with `x64 type` prompt as `mach.bat` set by default:
-> 1. you may need to choose and launch the type manually, such as `x86_x64 Cross Tools Command Prompt for VS 2017` in the Windows menu.)
+> 1. You may need to choose and launch the type manually, such as `x86_x64 Cross Tools Command Prompt for VS 2017` in the Windows menu.)
 > 2. `cd to/the/path/servo`
 > 3. `python mach build -d`