docs: updates docs to only support cypress < 10

Author: jordanpowell88

README.md

@@ -18,6 +18,8 @@ npm install --save-dev @cypress/browserify-preprocessor
 
 ## Usage
 
+### Note: This Plugin only works for Cypress < 10
+
 In your project's [plugins file](https://on.cypress.io/plugins-guide):
 
 ```javascript