Document creating specific managers in the README.md

This is related to https://github.com/Azure/azure-libraries-for-net/issues/200#issuecomment-371219101 to help users understand that they can go through specific managers rather than the "rollup" manager when using our libraries.

I took a look at our samples an some of the tests and most of them use the "rollup" Azure manager.

从 README.md 以及其中引用的 samples 和 tests 开始，了解这些库目前如何创建 manager。使用链接的 issue 评论作为背景，然后记录用户如何创建特定的 manager，而不是依赖 rollup manager。当 README.md 清楚地解释了特定 manager 的使用方式时，即视为完成。

由索引模型根据 Issue 内容生成。