Add MIGRATION.md

Author: paulbalandan

MIGRATION.md

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+# Migrating from Liaison CS Config Factory
+
+The following were the changes implemented in considering the transition to NexusPHP CS Config:
+
+## Added
+
+- Public methods `forProjects` and `forLibrary` were added to `Factory` class.
+- `forProjects` accepts no arguments and acts as the old `Factory::create`.
+- `forLibrary` accepts two required and two optional arguments and behaves as the old `Factory::createForLibrary`.
+- The `Nexus73` ruleset is added and meant for use in PHP 7.3+.
+- `declare(strict_types=1)` was added to all PHP files for additional type safety.
+
+## Changed
+
+- Root namespace of `Liaison\CS\Config` was renamed to `Nexus\CsConfig`.
+- `Factory::create` now returns an instance of `Factory` instead of `PhpCsFixer\Config`. This allows it
+to be chained with either `forProjects` or `forLibrary` public methods.
+
+## Removed
+
+- `Factory::createForLibrary` was removed. Use the public method `forLibrary` to get similar effect.
+- The `Liaison` and `CodeIgniter4` rulesets were removed.
+- The deprecated `BaseRuleset` class was removed.
+- Custom fixers were removed and moved to a new repository, [NexusPHP CS Fixers][1].
+
+[1]: https://github.com/NexusPHP/cs-fixers
+
+Please have a look at this library's [`.php_cs.dist`](.php_cs.dist) to have a glance of the new changes
+in action. You can also refer to the [README](README.md) for additional details.