arm64: stacktrace: Add description of stacktrace/common.h

Add brief description on how to use stacktrace/common.h to implement
a stack unwinder.

Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220726073750.3219117-7-kaleshsingh@google.com