update readme (fix #570)

README.md

@@ -61,6 +61,7 @@ Dependencies:
 + TensorFlow >= 1.2.0
 + Python bindings for OpenCV (Optional, but required by a lot of features)
 ```
+# install git, then:
 pip install -U git+https://github.com/ppwwyyxx/tensorpack.git
 # or add `--user` to avoid system-wide installation.
 ```