Add README file to the download zip

Description of new feature

Add a README file to the download zip.

The file could contain the following information:

The permalink for the network.
How to cite EnrichmentMap
Links to other resources like a FAQ.
A quick description of each file in the zip.
Instructions on how to load the data into EM-desktop.
The filter (cutoff) parameters used by the EM-service. (More advanced users ask to see this. Its also useful when loading into EM-desktop.)
Any warnings from the import step. For example if X number of ensembl genes could not be mapped we could say that.
Other useful information...

cytoscape / enrichment-map-webapp