Update raster extract readme file

[kklausmeyer]

I will create a fork with suggested text edits, but wanted to capture some edits here from Brian:

• Just a note to say that users will need the instruction to add the earthengine package if they do not already have it installed. https://developers.google.com/earth-engine/guides/python_install
• Also, make sure to add an instruction to users to be sure to be in the raster_extract folder when they run the test code (simple.py, for example)