DOC: Add core migration guide (#774)

* Add core migration guide
* Remove markdown migration tracker in favor of GitHub project

MIGRATION_TO_CORE.md

@@ -0,0 +1,28 @@
+# Migration From TF-Addons To TensorFlow Core
+
+### In-Progress & Previous Migrations:
+https://github.com/tensorflow/addons/projects/2/
+
+### Process 
+1. Create an issue in TensorFlow Addons for a candidate that you think should be 
+migrated. 
+2. The SIG will evaluate the request and add it to the `Potential Candidates` section 
+of our GitHub project.
+3. If it's agreed that a migration makes sense, an RFC needs to be written to discuss 
+the move with a larger community audience. 
+    * Additions which subclass Keras APIs should submit their migration proposals to 
+    [Keras Governance](https://github.com/keras-team/governance)
+    * Other additions should submit their migration proposals to 
+    [TensorFlow Community](https://github.com/tensorflow/community)
+4. If approved, a pull request must move the addition along with proper tests.
+5. After merging, the addition will be replaced with an alias to the core function 
+if possible.
+6. If an alias is not possible (e.g. large parameter changes), then a deprecation 
+warning will be added and eventual removed after 2 releases.
+
+
+### Criteria for Migration
+* The addition is widely used throughout the community
+* The addition is unlikely to have API changes as time progresses
+* The addition is well written / tested
+