thread safety up

### Problem:

[Thread safety](https://github.com/aws/aws-encryption-sdk-python?tab=readme-ov-file#thread-safety) section contains link to the [resources doc](https://boto3.amazonaws.com/v1/documentation/api/latest/guide/resources.html#multithreading-or-multiprocessing-with-resources) but aws-encryption-sdk-python does not use boto3 resources and use boto3 client. According to [clients doc](https://boto3.amazonaws.com/v1/documentation/api/latest/guide/clients.html#multithreading-or-multiprocessing-with-clients) it's thread-safe and could be used in a multithreaded application.

I read #560 and [https://github.com/aws/aws-encryption-sdk-python/issues/11#issuecomment-330716036](https://github.com/aws/aws-encryption-sdk-python/issues/11#issuecomment-330716036), there used irrelevant boto docs, because aws-encryption-sdk-python does not use boto3 resource, but client.

### Solution:

Review multithreaded guidelines and update doc.

从 README 中的 Thread safety 部分开始，将其当前的 resources 链接与 issue 中引用的 boto3 clients 多线程指南进行比较。更新文档，以反映 SDK 对 boto3 clients 的使用及其线程安全指南；完成的标准是链接和措辞准确。

由索引模型根据 Issue 内容生成。