Add TESTING.md with instructions about FakeWeb and curl

TESTING.md

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+# Discourse Developer Testing Guide
+
+Some notes about testing Discourse:
+
+## FakeWeb
+
+We use the [FakeWeb](https://github.com/chrisk/fakeweb) gem to fake external web 
+requests.
+For example, check out the specs on `specs/components/oneboxer`.
+
+This has several advantages to making real requests:
+
+* We freeze the expected response from the remote server.
+* We don't need a network connection to run the specs.
+* It's faster.
+
+So, if you need to define a spec that makes a web request, you'll have to record 
+the real response to a fixture file, and tell FakeWeb to respond with it for the 
+URI of your request.
+
+Check out `spec/components/oneboxer/amazon_onebox_spec.rb` for an example on 
+this.
+
+### Recording responses
+
+To record the actual response from the remote server, you can use curl and save the response to a file. We use the `-i` option to include headers in the output
+
+    curl -i http://en.m.wikipedia.org/wiki/Ruby > wikipedia.response
+
+If you need to specify the User-Agent to send to the server, you can use `-A`:
+
+    curl -i -A 'Mozilla/5.0 (iPhone; CPU iPhone OS 5_0_1 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9A405 Safari/7534.48.3' http://en.m.wikipedia.org/wiki/Ruby > wikipedia.response 
+    
+If the remote server is responding with a redirect, you'll need to fake both the 
+original request and the one for the destination. Check out the 
+`wikipedia.response` and `wikipedia_redirected.response` files in 
+`spec/fixtures/oneboxer` for an example. You can also consider working directly 
+with the final URL for simplicity.
+