doc: local development and tests info

README.md

@@ -69,6 +69,24 @@ It's also possible to set it to `warn` or `info`.
 
 The `extends` field is automatically added (or appended to) by the linter script to inject the iSP ruleset into your config.
 
+### Local Development
+
+1. Install `node 13.14.0` locally.
+2. Run `npm run build` to compile the `isp-functions`.
+3. Run `node ./entrypoint.js <PATH_TO_OPENAPI_SPEC>` to run the linter.
+
+The `build` command may need to be re-run to pick up changes made to some of the `isp-functions`.
+
+#### Tests
+
+```sh
+# Compile the isp-functions
+npm run build
+
+# Run test
+npm run test
+```
+
 ## License
 
 Copyright © 2020 iStreamPlanet Co., LLC