doc: add documentation for shared library libbitcoinconsensus

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+Shared Libraries
+================
+
+## bitcoinconsensus
+
+The purpose of this library is to make the verification functionality that is critical to Bitcoin's consensus available to other applications, e.g. to language bindings.
+
+### API
+
+The interface is defined in the C header `bitcoinconsensus.h` located in  `src/script/bitcoinconsensus.h`.
+
+#### Version
+
+`bitcoinconsensus_version` returns an `unsigned int` with the the API version *(currently at an experimental `0`)*.
+
+#### Script Validation
+
+`bitcoinconsensus_verify_script` returns an `int` with the status of the verification. It will be `1` if the input script correctly spends the previous output `scriptPubKey`.
+
+##### Parameters
+- `const unsigned char *scriptPubKey` - The previous output script that encumbers spending.
+- `unsigned int scriptPubKeyLen` - The number of bytes for the `scriptPubKey`.
+- `const unsigned char *txTo` - The transaction with the input that is spending the previous output.
+- `unsigned int txToLen` - The number of bytes for the `txTo`.
+- `unsigned int nIn` - The index of the input in `txTo` that spends the `scriptPubKey`.
+- `unsigned int flags` - The script validation flags *(see below)*.
+- `bitcoinconsensus_error* err` - Will have the error/success code for the operation *(see below)*.
+
+##### Script Flags
+- `bitcoinconsensus_SCRIPT_FLAGS_VERIFY_NONE`
+- `bitcoinconsensus_SCRIPT_FLAGS_VERIFY_P2SH` - Evaluate P2SH ([BIP16](https://github.com/bitcoin/bips/blob/master/bip-0016.mediawiki)) subscripts
+- `bitcoinconsensus_SCRIPT_FLAGS_VERIFY_DERSIG` - Enforce strict DER ([BIP66](https://github.com/bitcoin/bips/blob/master/bip-0066.mediawiki)) compliance
+
+##### Errors
+- `bitcoinconsensus_ERR_OK`
+- `bitcoinconsensus_ERR_TX_INDEX` - An invalid index for `txTo`
+- `bitcoinconsensus_ERR_TX_SIZE_MISMATCH` - `txToLen` did not match with the size of `txTo`
+- `bitcoinconsensus_ERR_DESERIALIZE` - An error deserializing `txTo`
+
+### Example Implementations
+- [node-libbitcoinconsensus](https://github.com/bitpay/node-libbitcoinconsensus) (Node.js Bindings)