Correcting yaml

putting `format: md_document` did not do anything for me, I believe the correct yaml field to output a github flavored markdown document is.

```
---
output:
  md_document:
    variant: markdown_github
---
```

The best way I can see to ensure the README.md gets regenerated would be to use a git post-commit hook that checks if the previous commit changed README.Rmd, if so run `render` on README.Rmd, stage the new README.md file and git commit --amend it to the just-ran commit.

Haven't actually tried to write the above hook to make sure it works though!
Merge pull request #108 from jimhester/patch-3

Correcting yaml