Adds CLI information to the README.

README.md

@@ -52,6 +52,36 @@ graph LR
 ![Example 2](http://www.sveido.com/mermaid/img/ex2.png)
 
 
+# mermaid CLI
+
+Installing mermaid globally (`npm install -g mermaid`) will expose the `mermaid` command to your environment, allowing you to generate PNGs from any file containing mermaid markup via the command line.
+
+**Note:** The `mermaid` command requires PhantomJS to be installed and available in your *$PATH*. You can specify it's location with the `-e` option.
+
+## Usage
+
+```
+$ mermaid --help
+
+Usage: mermaid [options] <file>...
+
+file    The mermaid description file to be rendered
+
+Options:
+  -s --svg          Output SVG instead of PNG (experimental)
+  -p --png          If SVG was selected, and you also want PNG, set this flag
+  -o --outputDir    Directory to save files, will be created automatically, defaults to `cwd`
+  -e --phantomPath  Specify the path to the phantomjs executable
+  -h --help         Show this message
+  -v --verbose      Show logging
+  --version         Print version and quit
+```
+
+## Known Issues
+
+- SVG output currently does some replacement on text, as mermaid's SVG output is only appropriate for browsers. Text color and background color is not yet replicated; please use PNGs for most purposes until this is resolved.
+- SVG output is decidedly non-standard. It works, but may cause issues in some viewers.
+
 
 # Credits
 Many thanks to the [d3](http://d3js.org/) and [dagre-d3](https://github.com/cpettitt/dagre-d3) projects for providing the graphical layout and drawing libraries! Thanks also to the [js-sequence-diagram](http://bramp.github.io/js-sequence-diagrams) project for usage of the grammar for the sequence diagrams.