Make sure installation instructions for VirtualArray are correct

[Nicklas Nordborg]

The current instructions are missing information about the need to create a formula. The plug-in will throw an exception about missing field 'reporter':

net.sf.basedb.core.ItemNotFoundException: Item not found: Unknown reporter field: reporter (you can add this field as an formula)
at net.sf.basedb.util.export.spotdata.Base1FieldConverter.getReporterField(Base1FieldConverter.java:102)
at net.sf.basedb.plugins.Base1PluginExecuter.exportData(Base1PluginExecuter.java:927)
at net.sf.basedb.plugins.Base1PluginExecuter.run(Base1PluginExecuter.java:669)
at net.sf.basedb.core.PluginExecutionRequest.invoke(PluginExecutionRequest.java:116)
at net.sf.basedb.core.InternalJobQueue$JobRunner.run(InternalJobQueue.java:510)
at java.lang.Thread.run(Unknown Source)

A formula needs to be created manually. The following instruction seems to be working:

1. Go to View -> Formulas page and click on the New button.
2. In the popup window enter the following information:

   Name: reporter
   Expressions: rep('id')
   Value type: Integer
   Avg. method: None
  // All other parameters can be left at their default values.

3. Save and then try running the VirtualArray plug-in again.

[Jari Häkkinen]

(In [2126]) Fixes #301. Added formula information and some minor updates.