Merge PR #3898: Circuit breaker high level explanation

.pending/improvements/sdk/926-circuit-breaker-

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+\#926 circuit breaker high level explanation

docs/spec/circuit-breaker/01_concepts.md

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+# Concepts
+
+The intention of the circuit breaker is to have a contingency plan for a
+running network which maintains network liveness. This can be achieved through
+selectively "pausing" functionality of specific modules on a running network.
+The circuit breaker is intended to be enabled through either:
+
+ - governance
+ - for emergencies a special subset of accounts selected by the state machine
+ - a transaction which proves the expected behaviour is broken
+
+## Pause state
+
+The basic pause state of any module simply disables all message routes to
+that module. Beyond that, it may be a appropriate for different modules to
+process begin-block/end-block in an altered "safe" way. 
+