Update introductory vignette

[Robinlovelace]

Typically to be called pkgname.Rmd for pkgdown, sfnetworks.Rmd in this case.

[Robinlovelace]

I see it's already been created: https://github.com/luukvdmeer/sfnetworks/blob/master/vignettes/sfnetworks.Rmd

Great work @luukvdmeer

[luukvdmeer]

Just as a reminder for myself: this is of high priority to wrap up milestone 1. The vignette should include:

• [x] Guidelines on installation
• [x] General philosophy/goal of the package
• [x] Explanation of the sfnetwork class
• [x] Guidelines on how to create an sfnetwork object
• [x] Overview of the sf methods for sfnetwork objects (summary of https://github.com/luukvdmeer/sfnetworks/issues/18#issuecomment-607896317)
• [x] Some simple examples that use both sf and tidygraph functions
• [x] Overview of what is still to come

[luukvdmeer]

Introductory vignette is added: https://github.com/luukvdmeer/sfnetworks/blob/master/vignettes/intro.Rmd