Create Agda.gitignore

Ignore .agdai files, they're binary files generated by typechecking .agda files.

Agda is an interactive proof assistant, home page here:
http://wiki.portal.chalmers.se/agda/pmwiki.php

`.agdai` files seem to be only documented here (and indirectly by googling):
http://wiki.portal.chalmers.se/agda/pmwiki.php?n=Main.PerformanceTips

A prominent repository using Agda on github:
https://github.com/HoTT/HoTT-Agda/
Their gitignore files:
https://github.com/HoTT/HoTT-Agda/blob/master/.gitignore
Merge pull request #738 from Blaisorblade/patch-1

Create Agda.gitignore