Update Documentation to show build steps

[oobayly]

This isn't an issue per se, but more of a placeholder for some additional information.

I was struggling to understand the process of rendering tiles, and came across some the pyrenderer repro by @stevo01. It does a great job of documenting and combining all the processes needed to generate the slippy tiles.

Another great resource is the Wiki, although the German version has more information and is easily translatable using Google Translate.

It would be great to have a link to this in the Readme.