Document the build process better

Add comments in gulpfile.js indicating how to add new files to
the build.
Add documentation for getting a development environment up and
running in the README.
Document each task defined in gulpfile.js in the README.

README.md

@@ -14,7 +14,20 @@ For local development, **node.js** build system is used.
 
 1. Install node.js
 1. From project folder run `npm install`
-1. To build JS and CSS use ` ./node_modules/gulp/bin/gulp.js`
+1. To build the JS and CSS files and start the configurator:
+    - With NW.js: Run `npm start`.
+    - With Chrome: Run `./node_modules/gulp/bin/gulp.js`. Then open `chrome://extensions`, enable
+    the `Developer mode`, click on the `Load unpacked extension...` button and select the `inav-configurator` directory.
+
+Other tasks are also defined in `gulpfile.js`. To run a task, use `./node_modules/gulp/bin/gulp.js task-name`. Available ones are:
+
+- **build**: Generate JS and CSS output files used by the configurator from their sources. It must be run whenever changes are made to any `.js` or `.css` files in order to have those changes appear
+in the configurator. If new files are added, they must be included in `gulpfile.js`. See the comments at the top of `gulpfile.js` to learn how to do so. See also the `watch` task.
+- **watch**: Watch JS and CSS sources for changes and run the `build` task whenever they're edited.
+- **dist**: Create a distribution of the app (valid for packaging both as a Chrome app or a NW.js app)
+in the `./dist/` directory.
+- **release**: Create NW.js apps for each supported platform (win32, osx64 and linux64) in the `./apps`
+directory.
 
 ## Authors
 

gulpfile.js

@@ -7,6 +7,31 @@ var del = require('del');
 var runSequence = require('run-sequence');
 var NwBuilder = require('nw-builder');
 
+
+// Each key in the *sources* variable must be an array of
+// the source files that will be combined into a single
+// file and stored in *outputDir*. Each key in *sources*
+// must be also present in *output*, whose value indicates
+// the filename for the output file which combines the
+// contents of the source files.
+//
+// Keys must be camel cased and end with either 'Css' or
+// 'Js' (e.g. someSourcesCss or someSourcesJs). For each
+// key, a build task will be generated named by prepending
+// 'build-' and converting the key to dash-separated words
+// (e.g. someSourcesCss will generate build-some-sources-css).
+//
+// Tasks with names ending with '-js' will be executed by the
+// build-all-js task, while the ones ending with '-css' will
+// be done by build-all-css. There's also a build task which
+// runs both build-all-css and build-all-js.
+//
+// The watch task will monitor any files mentioned in the *sources*
+// variable and regenerate the corresponding output file when
+// they change.
+//
+// See README.md for details on the other tasks.
+
 var sources = {};
 
 sources.css = [