Rewrite introduction paragraph in documentation

[djhoese]

I just had a coworker ask if I had a paragraph describing Satpy that he could include in a proposal for a project that will use Satpy. I pointed him to the documentation but quickly realized that the first paragraph of the sphinx docs home page is not a very good summary/description of Satpy as a whole. We/I should rewrite this. This is what it is currently:

Satpy is a python library for reading and manipulating meteorological remote sensing data and writing it to various image and data file formats. Satpy comes with the ability to make various RGB composites directly from satellite instrument channel data or higher level processing output. The pyresample package is used to resample data to different uniform areas or grids. Various atmospheric corrections and visual enhancements are also provided, either directly in Satpy or from those in the PySpectral and TrollImage packages.