Add draft vignette

[cforgaci]

@fnattino, I drafted the vignette. I suggest, we call the package data bucharest and include streets and river_centreline in it. For now, I disabled the code chunks, but once the data are in, we can re-enable them. That will also produce the plots in the rendered articles.

[fnattino]

Nice idea! Also pinging @vanlankveldthijs who is working on the packaged data as part of #2 . It would then be nice to include also the river_centerline to the packaged data (in an analogous way as this is done in the CRiSp package).

[fnattino]

Looks great @cforgaci! Indeed, when #27 is merged, we can finalize this.

[fnattino]

Hey @cforgaci , some other comments on the vignette - we can merge only after #31 as we depend on that bug fix in order to run the from_edge section.