Tune vulcanize doc for Mac-flavored npm

chrome/browser/resources/md_downloads/vulcanize_readme.md

Index: chrome/browser/resources/md_downloads/vulcanize_readme.md
diff --git a/chrome/browser/resources/md_downloads/vulcanize_readme.md b/chrome/browser/resources/md_downloads/vulcanize_readme.md
index dba53481551e593018b6bdf077cb7b510f8c4cda..2be2248ada9519cdbea8f3a6e47bdcd474ada76f 100644
--- a/chrome/browser/resources/md_downloads/vulcanize_readme.md
+++ b/chrome/browser/resources/md_downloads/vulcanize_readme.md
@@ -18,12 +18,21 @@ Vulcanization currently requires:
 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