Use RStudio project to declare files encoding

The documentation states that:

- RStudio is used for development
- source files are encoded in `ISO-8859-1`

Since `ISO-8859-1` encoding is not the usual encoding for R source files (UTF-8 or ASCII are mostly preferred), I think that the project could benefit by adding an `.Rproj` file declaring this encoding (see also https://github.com/InseeFr/Avionic/issues/1#issuecomment-656609164). This will avoid issues while opening the source files for R users who always work with UTF-8.

I'll do a PR.