simplify instructions using npx

Author: Kelly Selden

README.md

@@ -22,11 +22,10 @@ repository like Git and that you have no outstanding changes to commit before
 running this tool.
 
 ```bash
-npm install -g ember-test-helpers-codemod
 cd my-ember-app-or-addon
-ember-test-helpers-codemod --type=integration tests/integration
-ember-test-helpers-codemod --type=acceptance tests/acceptance
-ember-test-helpers-codemod --type=native-dom tests
+npx ember-test-helpers-codemod --type=integration tests/integration
+npx ember-test-helpers-codemod --type=acceptance tests/acceptance
+npx ember-test-helpers-codemod --type=native-dom tests
 ```
 
 ## Transformations
@@ -121,4 +120,4 @@ jscodeshift -t path/to/ember-test-helpers-codemod/lib/transforms/find.js tests
 
 Note that this will require all instances of `find`/`findAll` to have the correct `this` context, otherwise you will run
 into `Cannot read property 'querySelector' of undefined` exceptions, as `this.element` will not be defined. This can 
-happen outside of the main `test` function, for example inside of custom test helper functions.
+happen outside of the main `test` function, for example inside of custom test helper functions.