Add notes that "Run as Administrator is required"

[tdaniely]

> npm i -g node-nightly

C:\Users\REDACTED\AppData\Roaming\npm\node-nightly -> C:\Users\REDACTED\AppData\Roaming\npm\node_modules\node-nightly\cli.js
+ [email protected]
added 160 packages in 10.488s

> node-nightly

Downloading the nightly version, hang on...
{ Error: EPERM: operation not permitted, rename 'C:\Users\tdaniely\AppData\Roaming\npm\node_modules\node-nightly\node-v10.0.0-nightly20180213e782715d0a-win-x64' -> 'C:\Users\tdaniely\AppData\Roaming\npm\node_modules\node-nightly\node-nightly'
    at fs.renameSync (fs.js:784:18)
    at download.then._ (C:\Users\tdaniely\AppData\Roaming\npm\node_modules\node-nightly\index.js:45:9)
    at <anonymous>
  errno: -4048,
  code: 'EPERM',
  syscall: 'rename',
  path: 'C:\\Users\\tdaniely\\AppData\\Roaming\\npm\\node_modules\\node-nightly\\node-v10.0.0-nightly20180213e782715d0a-win-x64',
  dest: 'C:\\Users\\tdaniely\\AppData\\Roaming\\npm\\node_modules\\node-nightly\\node-nightly' }

Should be documented.