[ANNOUNCE] Add rexxiong(Jiashu Xiong) as new committer

[Keyong Zhou <zh...@apache.org>]

Hi Celeborn(-incubating) community,

I'm very excited to announce that recently we added rexxiong(Jiashu Xiong)
as our new committer!

rexxiong has kept contributing to Celeborn for near five months, mainly on
the support of Flink.
Looking forward that rexxiong will continue contributing to the project,
pushing Celeborn to the next level together with all contributors of the
community!

Also, we are looking forward to add more and more committers to our project
:)

Thanks!
Keyong Zhou