Tune vulcanize doc for Mac-flavored npm

chrome/browser/resources/md_downloads/vulcanize_readme.md

 # Vulcanizing Material Design downloads
 
 `vulcanize` is an npm module used to combine resources.  In order to make the
 Material Design downloads page sufficiently fast, we run vulcanize on the source
 files to combine them and reduce blocking load/parse time.
 
 ## Required software
 
 Vulcanization currently requires:
 
 - node.js: >= v4.4.2 (can be found with `node --version`)
 - npm: >= 1.3.10 (can be found with `npm --version`)
 - vulcanize: 1.14.8 (can be found with `vulcanize --version`)
 - crisper: 2.0.1 (can be found with `npm list -g crisper`)
 
 ## Installing required software
 
 For instructions on installing node and npm, see
 [here](https://docs.npmjs.com/getting-started/installing-node).
 
-Once you've installed npm, you can get `crisper` and `vulcanize` via:
+We recommend telling npm where to store downloaded modules:
 
 ```bash
-$ sudo npm install -g crisper vulcanize

[Dan Beam, 2016/04/05 15:39:04]

i've never liked that we needed to use `sudo` for this.

this should fix that.

+$ npm config set -g prefix "$HOME/node_modules"
 ```
 
+Then install `crisper` and `vulcanize` like this:
+
+```bash
+$ npm install -g crisper vulcanize
+```
+
+Ultimately, all that is required to run this script is that `crisper` and
+`vulcanize` are on your `$PATH`.
+
 ## Combining resources with vulcanize
 
 To combine all the CSS/HTML/JS for the downloads page to make it production
 fast, you can run the commands:
 
 ```bash
 $ chrome/browser/resources/md_downloads/vulcanize.py  # from src/
 ```
 
 This should overwrite the following files:
 
 - chrome/browser/resources/md_downloads/
  - vulcanized.html (all <link rel=import> and stylesheets inlined)
  - crisper.js (all JavaScript, extracted from vulcanized.html)
 
 ## Testing downloads without vulcanizing
 
 Build with "use_vulcanize=0" in your GYP_DEFINES to build downloads without
 vulcanizing.