[Windows] add installing lld-link.exe to the readme?

[jasonwilliams]

Developing Servo on Windows 10 can be painfully slow using the built-in linker.
Every build (even from the smallest change) can take around 30mins, i believe this is due to this bug:
rust-lang/rust#37543

Thanks to help from @retep998 ive had significant speed ups using lld-link instead, so should other Servo devs be setting this up?

Setup instructions

• Download lld-link from https://llvm.org/builds/
• Make sure lld-link.exe is on your sytstem path
• In your local ~/.cargo/config file set this (change the triple to match your setup)

[target.x86_64-pc-windows-msvc]
linker = "lld-link.exe"

Until the issue above is fixed should we recommend users install lld-link.exe instead?

Setup
Ryzen 7 1700 (8 Cores/16 Threads)

@larsbergstrom @vvuk @jdm @SimonSapin

[larsbergstrom]

@jasonwilliams I think we'd definitely take a change to the readme that points at this - anything that makes the dev experience better is 👍 from me!