Updates to note common issues.

README.md

@@ -47,7 +47,7 @@ Read more about it here in the [Rails Guides](https://guides.rubyonrails.org/con
 
 ### Rack Configuration
 
-NOTE: If you're running Rails, updating in `config/application.rb` should be enough.  There is no need to update `config.ru` as well.
+NOTE: If you're running Rails, adding `config/initializers/cors.rb` should be enough.  There is no need to update `config.ru` as well.
 
 In `config.ru`, configure `Rack::Cors` by passing a block to the `use` command:
 
@@ -109,6 +109,15 @@ A Resource path can be specified as exact string match (`/path/to/file.txt`) or
 
 ## Common Gotchas
 
+### Origin Matching
+
+When specifying an origin, make sure that it does not have a trailing slash.
+
+### Testing Postman and/or CURL
+
+* Make sure you're passing in an `Origin:` header.  That header is required to trigger a CORS response.
+* Make sure your origin does not have a trailing slash.
+
 ### Positioning in the Middleware Stack
 
 Positioning of `Rack::Cors` in the middleware stack is very important. In the Rails example above we put it above all other middleware which, in our experience, provides the most consistent results.