README.md: markdown formatting

README.md

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+## [bitcoin/libblkmaker](https://github.com/bitcoin/libblkmaker)
+
+### Dependencies:
+
+[Jansson 2.0](https://github.com/akheron/jansson/tree/v2.0) with 'long long' support
+
+### Example dependencies:
+
+[Jansson 2.1](https://github.com/akheron/jansson/tree/v2.1) (to read JSON from stdin)
+
+[libgcrypt](https://github.com/gpg/libgcrypt) (for SHA256)
+
+##### For usage, check out example.c.
+
+**Run**: `make example` to build it.
+
+**Note**:
+
+You must assign `blkmk_sha256_impl` to a function pointer
+
+```
+bool mysha256(void *hash_out, const void *data, size_t datasz)
+```
+
+**`hash_out` must be able to overlap with data!**
+
+Note that you should **NOT** roll ntime for data retrieved without explicitly
+checking that it falls within the template's limitations (mintime, maxtime,
+mintimeoff, and maxtimeoff).
+
+**Read the [BIP 23](https://github.com/bitcoin/bips/blob/master/bip-0023.mediawiki) specification in detail to
+understand how they work.**
+
+It is usually best to simply get more data as often
+as it is needed.
+
+For `blkmk_get_mdata`, you may specify that you intend to roll
+the ntime header exactly once per second past usetime - it will then set
+[*out_expires](blkmaker.c) such that the expiration occurs before you roll beyond any ntime
+limits.
+
+If you are rolling ntime at any rate other than once per second, you should NOT specify `can_roll_ntime` to `blkmk_get_mdata`, and must check that your
+usage falls within the explicit template limits yourself.