Added node installation.

README.md

@@ -17,10 +17,16 @@ API
 * `equal(a, b, tolerance)` tests image type objects for equality; accepts tolerance in pixels.
 * `diff(a, b)` performs an image diff on a and b, returning a - b.
 * `noConflict()` removes imagediff from the global space for compatibility, returning imagediff.
-* `imageDataToPNG(imageData, outputFile, (callback))` renders the imageData to png in outputFile, firing an optional callback upon save. (Node only)
+* `imageDataToPNG(imageData, outputFile, [callback])` (node only) renders the imageData to png in outputFile with optional callback.
+
+NodeJS
+------
+
+js-imagediff is available through the npm.  It uses [node-canvas](https://github.com/LearnBoost/node-canvas) which requires lib cairo to be installed.
+Install js-imagediff with `npm install -g imagediff`.
+
+### Command Line
 
-Command Line
----
 * `imagediff [-e|equal] [-t|tolerance VALUE] FILE_A FILE_B` tests equality of two image files with an optional tolerance, printing 'true' or 'false'.
 * `imagediff [-d|diff] FILE_A FILE_B OUTPUT_FILE` renders an imagediff between two files, saving as the output file.