Rbuildignore

At this moment there are `index.md` and `ergmgp.Rproj` in root directory that need to be added to `.Rbuildignore` because `R CMD check` complains:

```
(...)
N  checking top-level files
   Non-standard files/directories found at top level:
     ‘ergmgp.Rproj’ ‘index.md’
(...)
```
