Update README.md

README.md

@@ -5,7 +5,7 @@
 * Mailing list: [Google Groups](http://groups.google.com/group/terraform)
 
 Terraform is a tool for building and changing infrastructure
-safetly and efficiently.
+safely and efficiently.
 
 ## Developing Terraform
 
@@ -45,3 +45,36 @@ package by specifying the `TEST` variable. For example below, only
 
     $ make test TEST=./terraform
     ...
+
+### Acceptance Tests
+
+Terraform also has a comprehensive 
+[acceptance test](http://en.wikipedia.org/wiki/Acceptance_testing) 
+suite covering most of the major features of the built-in providers.
+
+If you're working on a feature of a provider and want to verify it
+is functioning (and hasn't broken anything else), we recommend running
+the acceptance tests. Note that we _do not require_ that you run or
+write acceptance tests to have a PR accepted. The acceptance tests
+are just here for your convenience. 
+
+**Warning:** The acceptance tests create/destroy/modify _real resources_,
+which may incur real costs. In the presence of a bug, it is technically
+possible that broken providers could corrupt existing infrastructure
+as well. Therefore, please run the acceptance providers at your own
+risk. At the very least, we recommend running them in their own private
+account for whatever provider you're testing.
+
+To run the acceptance tests, invoke `make testacc`:
+
+    $ make testacc TEST=./builtin/providers/aws TESTARGS='-run=VPC'
+    ...
+    
+The `TEST` variable is required, and you should specify the folder where
+the provider is. The `TESTARGS` variable is recommended to filter down
+to a specific resource to test, since testing all of them at once can
+take a very long time.
+
+Acceptance tests typically require other environment variables to be
+set for things such as access keys. The provider itself should error
+early and tell you what to set, so it is not documented here.