usage information in the README

Author: garethr

README.md

@@ -13,6 +13,20 @@ in the Open Policy Agent documentation.
 
 ## Usage
 
+```console
+$ conftest --help
+Test your configuration files using Open Policy Agent
+
+Usage:
+  conftest <file> [file...] [flags]
+
+Flags:
+      --fail-on-warn    return a non-zero exit code if only warnings are found
+  -h, --help            help for conftest
+  -p, --policy string   directory for Rego policy files (default "policy")
+      --version         version for conftest
+```
+
 ```console
 $ conftest deployment.yaml
 testdata/deployment.yaml

go.mod

@@ -9,7 +9,7 @@ require (
 	github.com/gobwas/glob v0.2.3 // indirect
 	github.com/hashicorp/go-multierror v1.0.0
 	github.com/inconshreveable/mousetrap v1.0.0 // indirect
-	github.com/logrusorgru/aurora v0.0.0-20181002194514-a7b3b318ed4e // indirect
+	github.com/logrusorgru/aurora v0.0.0-20181002194514-a7b3b318ed4e
 	github.com/open-policy-agent/opa v0.10.6
 	github.com/pkg/errors v0.8.1 // indirect
 	github.com/rcrowley/go-metrics v0.0.0-20181016184325-3113b8401b8a // indirect