create a README for the example notebooks folder

[mnky9800n]

This can describe the structure of the notebooks, like what order they should go in, and a basic summary of each.

[seismoChas]

Tell me what order you think they should go in. This would be the same order you think the notebooks should be introduced to students in a class.

[mnky9800n]

Well, in reality its probably better to not have a structure like that, but instead rely on the structure from the learning goals. Like the table in the paper.

[mnky9800n]

https://github.com/mnky9800n/Python-course-materials-for-seismology-students/blob/master/example_notebooks/README.md

I created a readme that includes the table and learning goals.

[seismoChas]

The readme is in the master... maybe you meant for it to be in the cartopy branch?