移除 sdk/php 目录，即将改用 git subtree
Squashed 'sdk/php/' content from commit 98865c1

git-subtree-dir: sdk/php
git-subtree-split: 98865c1b5924829ff54ca48230fca8250d6af4ca
Merge commit 'c0a0dad231efe20ae97bfaf210443aaa9bc1b275' as 'sdk/php'
PHP-SDK: update README.md content
在发行脚本中支持同步子树: xs-sdk-php