doc: add documentation for shared library libbitcoinconsensus

Author: Braydon Fuller

doc/README.md

@@ -58,6 +58,7 @@ The Bitcoin repo's [root README](https://github.com/bitcoin/bitcoin/blob/master/
 - [Translation Strings Policy](translation_strings_policy.md)
 - [Unit Tests](unit-tests.md)
 - [Unauthenticated REST Interface](REST-interface.md)
+- [Shared Libraries](shared-libraries.md)
 - [BIPS](bips.md)
 - [Dnsseed Policy](dnsseed-policy.md)
 

doc/shared-libraries.md

@@ -0,0 +1,41 @@
+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)