Add github-oauth config documentation, fixes #1405

doc/04-schema.md

@@ -593,6 +593,10 @@ The following options are supported:
 * **github-protocols:** Defaults to `["git", "https", "http"]`. A list of
   protocols to use for github.com clones, in priority order. Use this if you are
   behind a proxy or have somehow bad performances with the git protocol.
+* **github-oauth:** A list of domain names and oauth keys. For example using
+  `{"github.com": "oauthtoken"}` as the value of this option will use `oauthtoken`
+  to access private repositories on github and to circumvent the low IP-based
+  rate limiting of their API.
 * **cache-dir:** Defaults to `$home/cache` on unix systems and
   `C:\Users\<user>\AppData\Local\Composer` on Windows. Stores all the caches
   used by composer. See also [COMPOSER_HOME](03-cli.md#composer-home).