Add docs to Exobrain.pm / exobrain

[pjf]

Because Exobrain.pm is where people are most likely to land when they search on the CPAN, it should have a full set of docs attached to it. These may refer the user to Exobrain::FAQ, or Exobrain::Manual (if we create those), but it should give them enough to know what this is, and why they might use it.

The exobrain ubic configuration file may also be a landing point for users. I'll pop that in another ticket, because we want to tweak that anyway.

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