Add @sunxiaoguang to MAINTAINERS.md (tikv#4397)

* Title: Add @sunxiaoguang to MAINTAINERS.md

My name is Sun Xiaoguang from Zhihu, where I help make a BigTable like system focusd on OLTP that is using TiKV. I've been contributing to TiKV and the library ecosystem for around a year now. I first touched the TiKV codebase since around March 2018.

As part of my involvement with the project I've contributed several improvements to the TiKV Project:

* [Added raw batch put/get/delete/scan](pingcap/kvproto#244)
* [Changed `get_region` and `get_region_info` to use a common implementation](tikv#3521)
* [Print help message when starting tikv-ctl without argument](tikv#3531)
* [Clean up unused #[allow] directives](tikv#3553)

I also am the first and primary author of the [TiKV Rust Client](https://github.com/tikv/client-rust). You can see related PRs for that here:

* [TiKV Rust Client RFC](tikv/rfcs#7)
* [The initial version of Raw KV Implementation](tikv/client-rust#14)
* [Change `raw::Client::get` to return `Option<Value>`](tikv/client-rust#24)
* [Remove cf and batch_scan from `example/raw.rs`](tikv/client-rust#21)

I also made the first PR to the mock TiKV project: [A basic implementation of mock TiKV API surface](tikv/mock-tikv#1)

As part of my involvement, the TiKV team inquired if I was interested in becoming a maintainer for the project.

This PR adds me to the list of maintainers. I have read and understand the expectations of maintainers described in the `GOVERNANCE.md` file.

Signed-off-by: Xiaoguang Sun <sunxiaoguang@zhihu.com>

MAINTAINERS.md

@@ -20,7 +20,7 @@ Name | Email | Focus
 
 Name | Email | Focus
 ----|---|---
-
+[Xiaoguang Sun](https://github.com/sunxiaoguang) | [sunxiaoguang@zhihu.com](mailto:sunxiaoguang@zhihu.com) | Focus Areas: Client, Storage
 
 # Maintaining Organizations