Update README.md

README.md

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 # Terraform Assertion
-A simple module that asserts (during planning) that a condition is true.
+A simple module that asserts that a condition is true. The assertion is checked during planning, unless the condition depends on a value that will only be known during apply.
+
+The module forces Terraform to fail if the condition is false by attempting to call a non-existant external command. It uses the error message in the command name so that the resulting Terraform failure message is descriptive (although it is slightly confusing in the format; see the example below).
+
+## Usage
+```
+module "assertion_unix_only" {
+  source  = "Invicton-Labs/assertion/null"
+  
+  // The condition to ensure is TRUE
+  // In this example, assert that the operating system uses forward slashes for path separators
+  condition = dirname("/") == "/"
+  
+  // The error message to print out if the condition evaluates to FALSE
+  error_message = "This Terraform configuration can only be run on Unix-based machines."
+}
+```
+
+Output on Unix machines:
+```
+No changes. Your infrastructure matches the configuration.
+```
+
+Output on Windows machines:
+```
+Error: can't find external program "ERROR: This Terraform configuration can only be run on Unix-based machines."
+```