add a README

README

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+hs-cryptohash provides lots of different secure digest algorithms.
+
+All different cryptohashes contains the exact same API, for ease of
+switching between different hashes; this API is separated two folds:
+the incremental API and the one-pass API.
+
+= Incremental API
+
+The incremental API permits the user to compute the digest incrementally,
+with the overhead of having to box and unbox the context in and out
+the computing function.
+
+this API is made of: init, update and finalize; this is similar
+to how a digest algorithm works in other languages.
+
+= One Pass API
+
+The one pass API permits a single shot computation that doesn't have
+the overhead of boxing and unboxing at each step.
+
+this API is made of: hash and hashlazy. hash works on a strict bytestring,
+whereas hashlazy works on a lazy bytestring.