Document how to test expressions without adding them

Would be nice to document a good way to test out expressions before adding

I've been using [ripgrep](https://github.com/BurntSushi/ripgrep) to test

```
$ rg 'password\s+=\s*".*"$'
```

then adding

```
$ git secrets --add 'password\s+=\s*".*"$'
```

The issue names no documentation file, so first locate the repository's existing usage or testing documentation and review how command examples are organized. Document the ripgrep check alongside the corresponding git-secrets command, with enough context for users to test an expression before adding it.

Written by the indexing model from the issue text.