Add contributing docs

Reviewed By: danielbuechele

Differential Revision: D6976258

fbshipit-source-id: 71b3d0ab4a071ce0c4a9c1241cd6295aa6d52b8e

website/contents/contributing/testing.md

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+---
+path: "contributing/testing"
+title: "Testing"
+hasPlayground: false
+---
+
+# Testing
+
+Yoga tries to be as close as possible to chrome in its flexbox behaviour.
+To ensure this most of Yoga's test suite is automatically generateded from
+running the corresponding layout in chrome using a webdriver which then generates 
+C++ test which asserts that Yoga will produce matching outputs for that layout.
+
+## Running the Test Suite
+
+1. Yoga builds with [buck](https://buckbuild.com). Follow their documentation to get up and running.
+2. For testing Yoga relies on [gtest](https://github.com/google/googletest) as a submodule. After cloning Yoga run `git submodule init` followed by `git submodule update`.
+3. In a terminal from the root of your Yoga checkout run `buck test //:yoga`.
+
+## Adding a Test
+
+Instead of manually writing a test which ensures parity with web implementations
+of Flexbox we make use of a generated test suite. We use `gentest/gentest.rb` to
+generate this test suite. Write the html which you want to verify in Yoga and put
+it in the `gentest/fixtures` folder, such as the following.
+
+```html
+<div id="my_test" style="width: 100px; height: 100px; align-items: center;">
+  <div style="width: 50px; height: 50px;"></div>
+</div>
+```
+
+Run `gentest/gentest.rb` to generate test code and re-run `buck test //:yoga`
+to validate the behavior. One test case will be generated for every root `div`
+in the input html with the string in the `id` corresponding to the test name.
+
+You may need to install the latest watir gem (`gem install watir`) and
+[ChromeDriver](https://sites.google.com/a/chromium.org/chromedriver/) to
+run `gentest/gentest.rb` Ruby script.
+
+## Manual test
+
+For some aspects of Yoga we cannot generate a test using the test generation
+infrastructure described earlier. For these cases we manually write a test in 
+the `/tests` directory.