Update README with instructions

README.md

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 # solc-rust
 
-Rust bindings for the Solidity compiler.
+Rust bindings for the [Solidity] compiler. It exposes [Compiler Input and Output JSON] API.
+
+## Usage
+
+Add the dependency, as usual:
+
+```toml
+[dependencies]
+solc = { git = "https://github.com/axic/solc-rust" }
+```
+
+In your project use it as:
+```rust
+pub fn main() {
+    // Let input be a valid "Standard Solidity Input JSON"
+    let input = "{}";
+    let output = solc::compile(&input);
+    assert_ne!(output.len(), 0);
+}
+```
+
+## Maintainer(s)
+
+Alex Beregszaszi
+
+## License
+
+GPL-3.0
+
+[Solidity]: https://github.com/ethereum/solidity
+[Compiler Input and Output JSON]: https://solidity.readthedocs.io/en/latest/using-the-compiler.html#compiler-input-and-output-json-description