Add a docs/README.md overview page

Author: phil-opp

docs/migration/README.md

@@ -0,0 +1,6 @@
+# Migration Guides
+
+These guides explain how to migrate your project from older `bootloader` versions:
+
+- [Migration from `v0.9.X`](v0.9.md)
+- [Migration from `v0.10.X`](v0.10.md)