Update readme

Use the correct name: `parity-secp256k1`. Document that there's a submodule involved.

README.md

 [![Build Status](https://travis-ci.org/apoelstra/rust-secp256k1.png?branch=master)](https://travis-ci.org/apoelstra/rust-secp256k1)
 
-### rust-secp256k1
+### parity-secp256k1
 
-`rust-secp256k1` is a wrapper around [libsecp256k1](https://github.com/bitcoin/secp256k1),
+`parity-secp256k1` is a wrapper around [libsecp256k1](https://github.com/bitcoin/secp256k1),
 a C library by Peter Wuille for producing ECDSA signatures using the SECG curve
-`secp256k1`. This library
+`secp256k1`. It is a fork of [`rust-secp256k1`](https://github.com/rust-bitcoin/rust-secp256k1).
+ 
+This library
 * exposes type-safe Rust bindings for all `libsecp256k1` functions
 * implements key generation
 * implements deterministic nonce generation via RFC6979
@@ -13,3 +15,13 @@ a C library by Peter Wuille for producing ECDSA signatures using the SECG curve
 
 [Full documentation](https://www.wpsoftware.net/rustdoc/secp256k1/)
 
+#### Build
+
+Clone the repository. Run the following:
+
+```
+git submodule init
+git submodule update
+cargo check
+```
+