Add @sunxiaoguang to MAINTAINERS.md (#4397)
Add @sunxiaoguang to MAINTAINERS.md (#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](https://github.com/pingcap/kvproto/pull/244)
* [Changed `get_region` and `get_region_info` to use a common implementation](https://github.com/tikv/tikv/pull/3521)
* [Print help message when starting tikv-ctl without argument](https://github.com/tikv/tikv/pull/3531)
* [Clean up unused #[allow] directives](https://github.com/tikv/tikv/pull/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](https://github.com/tikv/rfcs/pull/7)
* [The initial version of Raw KV Implementation](https://github.com/tikv/client-rust/pull/14)
* [Change `raw::Client::get` to return `Option<Value>`](https://github.com/tikv/client-rust/pull/24)
* [Remove cf and batch_scan from `example/raw.rs`](https://github.com/tikv/client-rust/pull/21)

I also made the first PR to the mock TiKV project: [A basic implementation of mock TiKV API surface](https://github.com/tikv/mock-tikv/pull/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 <ac8c70efc34e705c6f908e5bc079f77a75790d36@zhihu.com>
BusyJay/tikv,c4pt0r/tikv,pingcap/tikv,pingcap/tikv,BusyJay/tikv,BusyJay/tikv,BusyJay/tikv