Add quickstart to the homepage

[koppor]

When I am using gradle, the homepage does not provide a hint to start the tool. There is no task spotBugs. When including the gradle pulgin, there is findBugsMain (which currently does not work - see spotbugs/spotbugs#250).

• Could you add somewhere that one has to execute findBugsMain?
• Could you add an example output?
• Could you add information how to apply the plugin fb-contrib. fb is not the same as spotbugs and so I wonder how they are compatible and how I can use these rules within a gradle setting.