Add Plovr as dependency and add note on plovr build target

Developer-Guide.md

@@ -30,6 +30,7 @@ The minimum requirements are:
 * Java 7 (JRE and JDK)
 * [PhantomJS](http://phantomjs.org/) 1.6 or higher
 * [Closure Linter](https://developers.google.com/closure/utilities/docs/linter_howto)
+* [Plovr](http://plovr.com/) - you can use a build target (see below) to download this
 
 The executables `git`, `gjslint`, `java`, `jar`, `python`, and `phantomjs`
 should be in your `PATH`.  To install the `regex` Python module, run `pip install regex`.
@@ -79,7 +80,10 @@ latter is a meta-target that basically runs `lint`, `build`, and `test`.
 
 The `serve` target starts the [Plovr](http://plovr.com/) web server. You'll
 need to start the Plovr server for running the examples and the tests. More
-information on that further down.
+information on that further down. If you do not have Plovr, you can download
+it with:
+
+    $ ./build.py plovr
 
 Other targets include `doc` and `integration-test`. The latter is the target
 used on Travis CI. See ol3's [Travis configuration file](https://github.com/openlayers/ol3/blob/master/.travis.yml).