MAINTAINERS: Add B field based on the linux kernel MAINTAINERs

Text was stolen from the linux kernel
This is thus identical to the kernel just a different more compact format.
I am very happy also to switch the file entirely to the format of the linux kernel maintainer list
if people prefer

This allows specifying a bug tracker. It would allow a maintainer to
use gitlab, github, or something else to keep track of bugs

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

MAINTAINERS

@@ -17,6 +17,8 @@ patches and related discussions.
 
 (L <address>) *Mailing list* that is relevant to this area
 (W <address>) *Web-page* with status/info
+(B <address>) URI for where to file *bugs*. A web-page with detailed bug
+              filing info, a direct bug tracker link, or a mailto: URI.
 
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