document why we did not use tracboat

tsa/howto/gitlab.mdwn

@@ -330,4 +330,23 @@ rotation periods.
 
 ## Alternatives considered
 
-<!-- include benchmarks and procedure if relevant -->
+### Migration tools
+
+ahf implemente the gitlab using his own home-made tools that talk to
+the GitLab and Trac API. but there's also [tracboat](https://github.com/tracboat/tracboat) which is
+designed to migrate from trac to GitLab.
+
+We did not use Tracboat because it uses gitlab's DB directly and thus
+only works with some very specific version. Each time the database
+schema changes at GitLab, Tracboat needs to port to it. We prefered to
+use something that talked with the GitLab API.
+
+We also didn't like the output entirely, so we modified it but still
+used some of its regular expressions and parser.
+
+We also needed to implement the "ticket movement" hack (with the legacy
+project) which wasn't implemented in Tracboat.
+
+Finally, we didn't want to do complete user migration, but lazily
+transfer only some users.
+