Make vulcanize.md more generic and remove specifics about node modules

docs/vulcanize.md

-# Vulcanizing Chrome Polymer UIs
-
-`vulcanize` is an npm module used to combine resources.  In order to make the
-Material Design downloads and history pages 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`)
-- uglifyjs: 2.4.10 (can be found with `uglifyjs --version`)
-- polymer-css-build: 0.0.6 (can be found with `npm list -g polymer-css-build`)
-
-## Installing required software
-
-For instructions on installing node and npm, see
-[here](https://docs.npmjs.com/getting-started/installing-node).
-
-We recommend telling npm where to store downloaded modules:
-
-```bash
-$ npm config set -g prefix "$HOME/node_modules"
-```
-
-Then install the required modules:
-
-```bash
-$ npm install -g crisper@2.0.1 vulcanize@1.14.8 uglifyjs@2.4.10 \
-    polymer-css-build@0.0.6
-```
-
-Ultimately, all that is required to run this script is that the node binaries
-listed above are on your $PATH.
-
-## Combining resources with vulcanize
-
-To combine all the CSS/HTML/JS for all pages which use vulcanize, making them
-production fast, you can run the command:
-
-```bash
-$ chrome/browser/resources/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)
-- chrome/browser/resources/md_history/
- - app.vulcanized.html
- - app.crisper.js
-
-## Testing downloads without vulcanizing
-
-Build with `use_vulcanize = false` in your gn args to build without vulcanizing.