Add a minifying files example

examples/minifying_files.md

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+Minifying files is a very common task for web developers. There are two
+common uses cases: 
+
+1. Create an unminified and minified file
+2. Generate a minified file from an unminifed file.
+
+Doing #2 is very easy with rake pipeline. Doing #1 is slightly more
+complicated. For these examples assume there is a `MinifyFilter` that
+actually does the work.
+
+Doing the first use case creates a problem. Filters are destructive.
+That means if you change one file (move it, rewrite) it will not be
+available for the next filters. For example, taking `app.js` and
+minifying it will remove the unminfied source from the pipeline. 
+
+You must first duplicate the unminified file then minify it. Here's an
+`Assetfile`
+
+```ruby
+input "source" do
+  match "application.js" do
+    # Duplicate the source file for future filters (application.js)
+    # and provide duplicate in "application.min.js" for minifcation
+    copy ["application.js", "application.min.js"]
+  end
+
+  match "application.min.js" do
+    filter MinifyFilter
+  end
+end
+
+output "compiled"
+```
+
+Now you have two files: `application.js` and `application.min.js`. You
+can use this same technique everytime you need to transform a file and
+keep the source around for future filters.