Initial commit for extreme current notebook

Initial commit for extreme current notebook, not complete as yet, also updates readme.md
@birdage 
All this files seems to me netcdf-classic, so you can avoid the loop with `MFDataset`
```python
from netCDF4 import MFDataset
url1 = 'http://dods.ndbc.noaa.gov/thredds/dodsC/data/adcp/46087/46087a2011.nc'
url2 = 'http://dods.ndbc.noaa.gov/thredds/dodsC/data/adcp/46087/46087a2012.nc'
nc = MFDataset([url1, url2])
```