Documentation of GBrowse to JBrowse migration

[ihh]

We need a tutorial/HOWTO showing how to migrate an existing GBrowse installation to JBrowse, including discussions of the pitfalls and caveats (e.g. Perl callbacks).

At first, this guide should describe the manual process (or use of the command-line migration script). Later, when there is a GUI for migration, the guide can be simplified.

[rbuels]

@scottcain do you have a feeling for how much need there is for GBrowse-to-JBrowse migration docs these days?