Test and document use of GraphQL-core with uvloop

In #141 it was discussed whether using a different event loop such as `uvloop` can speed up async use of GraphQL-core.

We should do some benchmarking and document how to replace the default event loop with a more efficient one. However, it should still be possible to use GraphQL-core as a pure-Python function, i.e. without depending on libraries that need to be compiled, such as `uvloop`.

從 issue #141 中的討論開始，檢視目前如何記錄 async GraphQL-core 的使用方式與 event loop 設定。對 uvloop 與預設的 event loop 進行基準測試，接著記錄 uvloop 的選用方式，同時保留在沒有編譯函式庫相依性的情況下使用 pure-Python 的能力。

由索引模型根據 Issue 內容生成。