Update .gitignore in addon/ to ignore README.md and LICENSE.md

Author: SergeAstapov

addon/.gitignore

@@ -25,3 +25,9 @@
 /package.json.ember-try
 
 .eslintcache
+
+# The authoritative copies of these live in the monorepo root (because they're
+# more useful on github that way), but the build copies them into here so they
+# will also appear in published NPM packages.
+/README.md
+/LICENSE.md