Document how to use file:/// URI to install from local miniCRAN

[andrie]

Pull request #58 includes some sample code in the vignette to demonstrate how to use the file:/// URI to point to a local miniCRAN repository.

We should in general improve the documentation to demonstrate this, e.g:

• vignette
• in makeRepo()
• in README
• in package level documentation

Also, create documentation with instructions on how to expose a miniCRAN via HTTP, to act like a "real" mirror.

[benmarwick]

That sounds great, more docs along these lines will make this package very user friendly.

[achubaty]

• addLocalPackage (new function to add a local package #23) needs documentation/examples.
• I have a version of a document (and script) that provides instructions for setting up http miniCRAN server using nginx, but I haven't updated it since Ubuntu 14.04, so I would need help updating and testing. The instructions really should provide https instructions.

[vnijs]

FYI I had some trouble trying to install from a local directory on macOS using "file:///". Instead, using file.path("file:", repo_dir) works fine on linux and macOS.

Here "repo_dir" is the base directory and the package are in, for example, file.path(repo_dir, "src/contrib")