Add doc for mocha-test.

docs/mocha-test.md

@@ -0,0 +1,17 @@
+# Mocha Test
+
+Greenworks uses [Mocha](http://visionmedia.github.io/mocha/) framework to test
+the steamworks APIs.
+
+Since Greenworks is interacting with Steamworks, you need to create a
+`steam_appid.txt` file with a valid Steam Game Application ID
+(your own AppID or Steamworks example AppID) in the
+`<greenworks_src_dir>/test` directory, in order to run the tests.
+
+```shell
+cd <greenworks_src_dir>
+
+./test/run-test
+```
+
+See [how to find the application ID for a Steam Game](https://support.steampowered.com/kb_article.php?ref=3729-WFJZ-4175).