Update build instructions

We don't need to separately build the library and the executable anymore.

Author: fitzgen

CONTRIBUTING.md

@@ -42,19 +42,12 @@ issue, provide us with:
 
 ## Building
 
-To build `bindgen`:
+To build the `bindgen` library and the `bindgen` executable:
 
 ```
 $ cargo build
 ```
 
-To build the `bindgen` executable:
-
-```
-$ cd bindgen/bindgen
-$ cargo build
-```
-
 If you installed multiple versions of llvm, it may not be able to locate the
 latest version of libclang. In that case, you may want to either uninstall other
 versions of llvm, or specify the path of the desired libclang explicitly: