Add README file to the download zip

[mikekucera]

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...

[chrtannus]

It looks good--sample:
README.md