Updating README with working jekyll command

This is a very minor thing, but I realised when trying to get started that the command for running `jekyll` is outdated - namely, I got the following errors:

```
jekyll --auto --server --url localhost
Deprecation: The --server command has been replaced by the 'serve' subcommand.
Deprecation: The switch '--auto' has been replaced with '--watch'.
Deprecation: The 'url' setting can only be set in your config files.
```

This is just a slight improvement to the README so that people can get up and running faster - I hope this helps.

On a side note - great job James, this is a really nice project!