Add migration notes for shared libraries

Signed-off-by: methylDragon <[email protected]>

Migration.md

@@ -22,6 +22,9 @@ but with improved human-readability..
 - Header files under `ignition/...` are deprecated and will be removed in future versions.
   Use `gz/...` instead.
 
+* The shared libraries have `gz` where there used to be `ignition`.
+  * Using the un-migrated version is still possible due to tick-tocks, but will be removed in future versions.
+
 - **sdf/Types.hh**: The `Inertia` class has been deprecated. Please use the
    `Inertial` class in the `gz-math` library.