MAINT: add gitlab to github migration guide

MIGRATION.md

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+# Instructions for migrating from git.ligo.org to github.com
+
+We hope that migrating development from [git.ligo.org](https://git.ligo.org/lscsoft/bilby) to [GitHub](https://github.com/bilby-dev/bilby) will be relatively painless.
+This document provides instructions for common tasks.
+
+## `main` vs `master`
+
+This is not specifically a difference between Gitlab and GitHub, but we took this opportunity to make the [switch](https://www.theserverside.com/feature/Why-GitHub-renamed-its-master-branch-to-main).
+New PRs will automatically be into `main` and new clones will automatically checkout `main`.
+If you have existing clones (and muscle memory), make sure you always use `main` as the starting point for new branches.
+
+## Migrating development to GitHub:
+
+- [create a fork of github.com/bilby-dev/bilby if you don't already have one](https://github.com/bilby-dev/bilby/fork)
+- if git.ligo.org/lscsoft/bilby is your current origin remove `git remote remove origin`
+- make github the new origin `git remote add origin [email protected]:bilby-dev/bilby.git`
+- add your fork on github as a remote `git remote add mine [email protected]:${ME}/bilby.git`
+- checkout and update `main` `git checkout origin/main && git pull`
+- checkout the new feature branch `git checkout -b FEATURE-BRANCH-NAME`
+- push your feature branch to your fork `git push --set-upstream mine FEATURE-BRANCH-NAME`
+
+## Migrating an MR to GitHub:
+
+This largely follows the above instructions for migrating development.
+
+- [create a fork of github.com/bilby-dev/bilby if you don't already have one](https://github.com/bilby-dev/bilby/fork)
+- checkout your feature branch `git checkout FEATURE-BRANCH-NAME`
+- add your fork on github as a remote `git remote add mine [email protected]:${ME}/bilby.git`
+- push your feature branch to your fork `git push --set-upstream mine FEATURE-BRANCH-NAME`
+- open a PR to [github.com/bilby-dev/bilby](https://github.com/bilby-dev/bilby/compare)
+- copy the description and summarise relevant discussion
+- close the MR on [git.ligo.org](https://git.ligo.org/lscsoft/bilby/-/merge_requests)
+
+## Migrating an issue to GitHub:
+
+This should not be necessary, we plan to automatically bulk migrate all issues (open and closed.)