Readme: Note about native extensions, how to run/test (#134)

Signed-off-by: Arjun Sreedharan <asreedharan@vmware.com>

README.md

@@ -43,9 +43,36 @@ file that looks like the following:
 To package this buildpack for consumption:
 
 ```
-$ ./scripts/package.sh
+$ ./scripts/package.sh --version <version-number>
 ```
 
+This will create a `buildpackage.cnb` file under the `build` directory which you
+can use to build your app as follows:
+`pack build <app-name> -p <path-to-app> -b <path/to/node-engine.cnb> -b build/buildpackage.cnb`
+
 ## `buildpack.yml` Configurations
 
 The NPM Install buildpack does not support configurations using `buildpack.yml`.
+
+
+## Run Tests
+
+To run all unit tests, run:
+```
+./scripts/unit.sh
+```
+
+To run all integration tests, run:
+```
+/scripts/integration.sh
+```
+
+## Stack support
+
+For most apps, the NPM Install Buildpack runs fine on the [Base
+builder](https://github.com/paketo-buildpacks/stacks#metadata-for-paketo-buildrun-stack-images).
+But when the app requires compilation of native extensions using `node-gyp`,
+the buildpack requires that you use the [Full
+builder](https://github.com/paketo-buildpacks/stacks#metadata-for-paketo-buildrun-stack-images).
+This is because `node-gyp` requires `python` that's absent on the Base builder,
+and the module may require other shared objects.