changes to download page

[dsetrakyan]

Guys,

Given numerous questions asking about the differences between Apache Ignite
and GridGain Community Edition releases, I have added a link and
explanation to the Apache Ignite download page

Please review the text and let me know if it is kosher from ASF standpoint.
Will be happy to change it, if anything is wrong.

https://ignite.incubator.apache.org/download.html

Thanks,
D.