add instructions

Author: Kelly Selden

README.md

@@ -1,3 +1,14 @@
 # ember-cli-update-codemods-manifest
 
 Master list of codemods and their instructions used by [ember-cli-update](https://github.com/ember-cli/ember-cli-update)
+
+Tips for changing this list:
+
+1. Make the desired change to the manifest.json here.
+1. Fork [ember-cli-update](https://github.com/ember-cli/ember-cli-update).
+1. Do a code search for the URL containing "ember-cli-update-codemods-manifest".
+1. Change it to link to your changes from step 1.
+  * This will look like "https://raw.githubusercontent.com/ember-cli/ember-cli-update-codemods-manifest/vX/manifest.json" => "https://raw.githubusercontent.com/your-username/ember-cli-update-codemods-manifest/your-branch/manifest.json".
+1. Submit the change from step 4 as a pull request to [ember-cli-update](https://github.com/ember-cli/ember-cli-update). The purpose is to run the test suite.
+1. Fix any test failures resulting from step 5.
+1. If your change from step 1 is a new codemod instead of an update or fix to an existing, try and add a new test case to [test/fixtures/codemod](https://github.com/ember-cli/ember-cli-update/tree/master/test/fixtures/codemod). Otherwise, this step is not necessary.