docs: add Spectral CLI usage step (#12)

Author: Enda

spectral/README.md

@@ -4,6 +4,8 @@ A custom ruleset for [Spectral](https://stoplight.io/open-source/spectral/) foll
 
 ## Using
 
+> [npm](https://docs.npmjs.com/) is required to use Spectral.
+
 To use this Spectral ruleset add the following to your `.spectral.yaml`:
 
 ```yaml
@@ -12,6 +14,12 @@ rules:
 	# add your custom rules here
 ```
 
+Run the Spectral CLI: 
+
+```shell
+npx @stoplight/spectral ./path/to/openapi.yaml
+```
+
 ## Rules
 
 The RHOAS ruleset extends the Spectral built-in "oas" ruleset (except `operation-tags`, `openapi-tags`). You can see the full list of rules from that ruleset [here](https://meta.stoplight.io/docs/spectral/docs/reference/openapi-rules.md)
@@ -241,4 +249,4 @@ Validate OpenAPI files using the uncompiled ruleset:
 ```shell
 yarn build
 yarn spectral-lint ./examples/openapi-valid.yaml
-```
+```