[grpc-health-check] Documentation & problem with published package

### Problem description

There is no documentation around how `grpc-health-check` library should be used.

I have tried to use it based on existing test cases, but there is major issue with latest published version `1.6.6`:

The "main" file defined in `package.json` file is `src/node/index.js` [[here](https://github.com/grpc/grpc-node/blob/990f082f0094f32cc9e1d8f272886955e98afc9c/packages/grpc-health-check/package.json#L30)] while there is no such file.

Hence, we can't have `require('grpc-health-check')` because it will throw `Cannot find module 'grpc-health-check'` error. 

It should be used like `require('grpc-health-check/health')`. 



But it can be resolved easily by fixing that line to point to `health.js` and publish it as `1.6.7` or something.



### Reproduction steps

```bash

mkdir /tmp/test && cd /tmp/test

npm init

npm i grpc-health-check@1.6.6

node -e "require('grpc-health-check')"

```

### Environment

 - Any node version / but i've tested on 8.8.1

clang 3.8.0-2ubuntu4]

 - grpc-health-check@1.6.6



### Additional context

I'd be happy to raise PRs to fix that and also add documentation if this is not deprecated or sth.

Also wondering why link to repository in npmjs.com is pointing to https://github.com/grpc/grpc/ 🤔 

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