Document support for GitHub token authentication (#177)

* Added documentation for #84

* fixed spacing issue

* Uppercase

* Uppercase

* Remove trailing spaces

* Update `README.md`

* Remove remaining trailing space

---------

Co-authored-by: Edmund Turbin <edmund.turbin@pantheon.io>
Co-authored-by: Daniel Bachhuber <daniel@bachhuber.co>
Co-authored-by: Daniel Bachhuber <daniel.bachhuber@automattic.com>

README.md

@@ -151,6 +151,13 @@ reference breaks.
 When installing a .zip file, WP-CLI extracts the package to
 `~/.wp-cli/packages/local/<package-name>`.
 
+If Github token authorization is required, a GitHub Personal Access Token
+(https://github.com/settings/tokens) can be used. The following command
+will add a GitHub Personal Access Token to Composer's global configuration:
+composer config -g github-oauth.github.com <GITHUB_TOKEN>
+Once this has been added, the value used for <GITHUB_TOKEN> will be used
+for future authorization requests.
+
 **OPTIONS**
 
  <name|git|path|zip>

src/Package_Command.php

@@ -188,6 +188,13 @@ public function browse( $_, $assoc_args ) {
  * When installing a .zip file, WP-CLI extracts the package to
  * `~/.wp-cli/packages/local/<package-name>`.
  *
+ * If Github token authorization is required, a GitHub Personal Access Token
+ * (https://github.com/settings/tokens) can be used. The following command
+ * will add a GitHub Personal Access Token to Composer's global configuration:
+ * composer config -g github-oauth.github.com <GITHUB_TOKEN>
+ * Once this has been added, the value used for <GITHUB_TOKEN> will be used
+ * for future authorization requests.
+ *
  * ## OPTIONS
  *
  * <name|git|path|zip>