Use RStudio project to declare files encoding

[RLesur]

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 #1 (comment)). This will avoid issues while opening the source files for R users who always work with UTF-8.

I'll do a PR.

[FLimousin]

I'll see about that when i come back from holidays