Added notes regarding running the test suite to the README

I like the idea of adding the info about testing into the README!

However, the test suite will **wipe out the entire cluster**, so it's preferable to run a different cluster for tests.

There's a special extension for that in `elasticsearch-extensions`, and a Rake task like this one:

```
TEST_CLUSTER_COMMAND=/tmp/builds/elasticsearch-2.0.0-SNAPSHOT/bin/elasticsearch TEST_CLUSTER_NODES=1 bundle exec rake test:cluster:start
```

I'll add it to the README.