Update testing instructions

TESTING.md

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+## Running the mustache.js Test Suite
+
+The mustache.js test suite uses the [RSpec](http://rspec.info/) testing
+framework. In order to run the tests you'll need to install [Ruby](http://ruby-lang.org/)
+as well as the `rake`, `rspec` (>=2), and `json` [RubyGems](http://rubygems.org/).
+
+### How to install Ruby and the required gems from source
+
+Make sure you have the required tools to compile it:
+
+    $ apt-get install build-essential libssl-dev libreadline5-dev zlib1g-dev
+
+Download and extract the Ruby source, and install it:
+
+    $ wget ftp://ftp.ruby-lang.org/pub/ruby/stable-snapshot.tar.gz
+    $ tar xvzf stable-snapshot.tar.gz
+    $ cd ruby
+    $ ./configure && make && make install
+
+Download and extract RubyGems, and install it:
+
+    $ wget http://production.cf.rubygems.org/rubygems/rubygems-1.8.12.tgz
+    $ tar xzvf rubygems-1.8.12.tgz
+    $ cd rubygems-1.8.12
+    $ ruby setup.rb
+
+If you want to update RubyGems:
+
+    $ gem update --system
+
+Install the required gems:
+
+    $ gem install rake rspec json
+
+That's it!
+
+### How to run the tests
+
+The mustache.js test suite currently uses 4 different JavaScript runtime engines
+to maximize portability across platforms and browsers. They are:
+
+  * node
+  * SpiderMonkey (Mozilla, Firefox)
+  * JavaScriptCore (WebKit, Safari)
+  * Rhino (Mozilla, Java)
+
+When the test suite runs it will automatically determine which platforms are
+available on your machine and run on all of them. The suite must run on at least
+one platform in order to succeed.
+
+Once you have at least one JavaScript platform installed, you can run the test
+suite with the following command:
+
+    $ rake
+
+### How to create a test
+
+All test files live in the spec/_files directory. To create a new test:
+
+  * Create a template file `somename.mustache`
+  * Create a javascript file with data and functions `somename.js`
+  * Create a file the expected result `somename.txt`