Add description of how to run the tests to README.md

Fixes #42

Author: fitzgen

README.md

@@ -58,6 +58,12 @@ $ LIBCLANG_PATH=path/to/clang-3.9/build/lib \
     cargo build
 ```
 
+## Running Tests
+
+```
+$ make test
+```
+
 # Command Line Usage
 
 There are a few options documented when running `./bindgen --help`. Other