Updating the core README with info about what the core is, how it works and implememtation status.

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 external_avalanche_entropy
 ==========================
 
-Entropy collector and provider for an external avalanche noise based entropy source.
+Entropy provider core for an external avalanche noise based entropy source.
+
+## Functional Descriptopn ##
+
+This core samples noise provided on an input pin. The noise is expected
+to be 'digital' that is fairly rapidly move from voltage levels
+matching ones and zeros as handled by the digital process used to
+implement the core.
+
+The noise is sampled with double registers. Then phase detection is
+applied to find positive flanks. The core contains a free running clock
+(clocked at the provided core clock frequency). When a positive flank in
+the noise is detected, the LSB of the clock is sampled and added to a
+shift registers. When at least 32 bits has been collected, the result is
+presented as entropy available to any entropy consumer connected to the
+core.
+
+The core also includes a delta time counter. This counter is used for
+testing of the core and is available via the API.
+
+The fact that the core uses the flank of the to drive the entropy bit
+generation, but that the timing between the flanks means that if
+the noise source have a bias for zero or one state does not affect which
+entropy bits are generated.
+
+
+## Implementation Status ##
+
+The core has been tested with several revisions of the Cryptech
+avalanche noise board. The core has been implemented in Altera
+Cyclone-IV and Cyclone-V devices as well as in Xilinx Spartan-6
+devices. The core clock frequency used has been 25 MHz, 33 MHz and 50
+MHz.
+
+The generated entropy has been extensively tested and found to be
+generating entropy with good quality.