Moved Nitro submodule ahead and updated README

The `nitro` submodule was behind. After building and installing Nitro, I was getting the following traceback:

```
Traceback (most recent call last):
  File "examples/t.py", line 1, in <module>
    from pynitro import NitroSocket, NitroFrame
  File "/home/christian/src/nitro-python/pynitro/__init__.py", line 1, in <module>
    from pynitro import *
ImportError: /home/christian/src/nitro-python/pynitro/pynitro.so: undefined symbol: nitro_frame_clear
```

I also fleshed out the `README` with some installation instructions.