DOC: Add migration documentation (#561)

* Add migration documentation

* Update migrations

* Format

Author: seanpmorgan

MIGRATED_TO_CORE.md

@@ -0,0 +1,8 @@
+# Migrated To TensorFlow Core
+
+| Function | Status | Last TFA Version | First TF Version | Relevant PRs |  
+|:-------- |:------ |:---------------- |:---------------- |:-------------|
+| [metrics.F1Score](https://github.com/tensorflow/addons/blob/r0.5/tensorflow_addons/metrics/f_scores.py#L255) & [metrics.FBetaScore](https://github.com/tensorflow/addons/blob/r0.5/tensorflow_addons/metrics/f_scores.py#L26) | In Progress  |  |  | [TF-31818](https://github.com/tensorflow/tensorflow/pull/31818) & [TFA-430](https://github.com/tensorflow/addons/issues/430) | 
+| [utils.register_keras_custom_object](https://github.com/tensorflow/addons/blob/r0.5/tensorflow_addons/utils/keras_utils.py#L23) | In Progress |  |  | [TFA-438](https://github.com/tensorflow/addons/issues/438) | 
+| [activations.gelu](https://github.com/tensorflow/addons/blob/r0.5/tensorflow_addons/activations/gelu.py#L30) | Proposed |  |  | [TFA-550](https://github.com/tensorflow/addons/issues/550)  | 
+