Initial commit for extreme current notebook

Initial commit for extreme current notebook, not complete as yet, also updates readme.md
This is actually something I changed in OWSLib 0.8.8 to fix a CSW bug. https://github.com/geopython/OWSLib/commit/a14c4020301ba91fd029c976911cfbfe0aec28c8#diff-47f8076511050daa2f27bcd8796fb2baL171

.constraints now returns a list of Constraint objects (https://github.com/geopython/OWSLib/blob/master/owslib/ows.py#L149).  @birdage if you update to OWSLib 0.8.8 you will need to account for this.