docs: Clean up contributing

- Simplify instructions 
- Follow 80 character line limits
- Add ##Testing section
- Move link URLs to bottom
What do you think about syntax highlighting the terminal examples?

```
```sh
# Comment
$ command
```

In this case:

```sh
# Run the tests.
$ npm test

# Run the regression tests.
$ npm run regression-test

# See the test coverage report.
$ npm run cover
```

Then later:

```sh
# From the project's root directory:
$ gcloud preview datastore create-indexes regression/data/
```