proc: update MAINTAINERS with proc.txt

Turns out that /proc has official documentation and people even trying
to keep it uptodate.

Link: http://lkml.kernel.org/r/20181116134630.GA8004@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

MAINTAINERS

@@ -12075,6 +12075,7 @@ S:	Maintained
 F:	fs/proc/
 F:	include/linux/proc_fs.h
 F:	tools/testing/selftests/proc/
+F:	Documentation/filesystems/proc.txt
 
 PROC SYSCTL
 M:	Luis Chamberlain <mcgrof@kernel.org>