MAINTAINERS: add RELAY entry

RELAYFS was originally developed by Tom Zanussi and Karim Yaghmour in
2005[1]. Jens Axboe converted it from filesystem into a generic API in
2006[2] and made it widely known through the notable I/O tracing tool
blktrace. In the decade, there remain a few users scatterred across
different subsystems, like recently added wifi commit[3] that is an
example to show how to communicate between users and kernel. Last year
I've already done some maintenance and added/corrected some diagnostic
counters.

At Tencent, we internally maintain RELAY as one of most crucial components
of network observibility platform which was shared a bit at LPC 2025[4][5]
and hopefully will be published in the paper this year. RELAY has proven
highly efficient due to its inherent design essence. This design becomes
the indispensable way to build a 7x24 platform monitoring various hot
paths even without any selectively sampling (yes, sampling is commonly
used to avoid the overall performance degradation). One of the
recommended usages is to use its zerocopy function relay_reserve() to
transfer data in a raw format that can be recognized and parsed by the
corresponding application to userspace without introducing heavy locks and
complicated logic that appears in other types of approaches, like printk.
More details can be discovered by reading through the Documentation :)

Credits are given to the all the contributors and reviewers for
RELAY/RELAYFS in the past and future! Many thanks!

[1]: commit e82894f84dbb ("[PATCH] relayfs")
[2]: commit b86ff981a825 ("[PATCH] relay: migrate from relayfs to a generic relay API")
[3]: commit c1bf6959dd81 ("wifi: ath11k: Register relayfs entries for CFR dump")
[4]: https://lpc.events/event/19/contributions/2055/
[5]: https://lpc.events/event/19/contributions/2010/

Link: https://lkml.kernel.org/r/20260301020902.56476-1-kerneljasonxing@gmail.com
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Jens Axboe <axboe@kernel.dk>
Cc: Andriy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Tom Zanussi <zanussi@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>