Usage in combination with other gems

Updated the readme with info about use in combination with other gems. This will hopefully help solve similar issues as the one I was having.

See https://github.com/jekyll/jekyll-sitemap/pull/11#issuecomment-42773286

```shell
$ gem query -n jekyll --local

*** LOCAL GEMS ***

jekyll (2.0.3, 1.5.1)
jekyll-redirect-from (0.4.0, 0.3.1)
jekyll-sitemap (0.4.1, 0.2.0)
```

Faulty `_config.yml` that makes `sitemap.xml` contain redirect paths:
```
…
gems: ['jekyll-redirect-from', 'jekyll-sitemap']
```

Working `_config.yml`:
```
…
gems: ['jekyll-sitemap', 'jekyll-redirect-from']
```