Add a migration guide

[cmungall]

We should add a guide (or just a few lines to the README) that descibes how to migrate an older linkml repo to cookiecutter

there is no one size fits all approach here. Approaches vary from:

1. abandon old repo and create a new repo
   • disadvantages:
     • history broken (may not matter with small projects)
     • issues forked (can be migrated one by one using gh interface)
2. remove all files from current repo. create new repo using cc. copy all files across using cp -pr. git add -a
   • similar to above but issues preserved
3. create a new repo using cc
   • do not push
   • use new repo as an exemplar
   • manually rm and mv files in the existing repo to match the template
   • this is more work than 1/2 but should preserve history/issues
   • note that if you miss a file or put something in the wrong place it could lead to hard to debug makefile issues