fix: Move migration into a more generic file

Author: trodrigues

migration_from_2_x.md renamed to MIGRATION.md

@@ -1,3 +1,7 @@
+# Migration to contentful.js 3.x from previous versions
+
+__(March 8th, 2016)__
+
 contentful.js 3.x was a major rewrite, with some API changes. While the base functionality remains the same, some method names have changed, as well as some internal behaviors.
 
 The two major changes you should look out for are the method renamings, and the way link resolution works.

README.md

@@ -114,7 +114,7 @@ You can check the changelog on the [releases](https://github.com/contentful/cont
 
 contentful.js 3.x was a major rewrite, with some API changes. While the base functionality remains the same, some method names have changed, as well as some internal behaviors.
 
-See the [migration guide](migration_from_2_x.md) for more information.
+See the [migration guide](MIGRATION.md) for more information.
 
 ## Support