Readme & decode demo updates

[mapsam]

This fixes the /demo/decode.cpp to work with the 2.x branch. It's a little clumsy, and still relies on the branch of geometry.hpp that hasn't been merged yet. I had trouble writing a proper geometry type reader, but we could theoretically piggy-back on the work over at mapbox/geometry.hpp#56

Updates build instructions as well in the README so others can replicate. #34

cc @mapbox/core-tech @joto

[mapsam]

Also noting that the vtzero decode demo is pretty solid, and is likely more reliable than this one. Curious if the demo is necessary? Right now it feels a little redundant, and doesn't really highlight the benefits of using lib vector-tile.

[springmeyer]

Right now it feels a little redundant, and doesn't really highlight the benefits of using lib vector-tile.

Yes, the demo code pre-dates vtzero and the 2.x branch of this repo. It is not critical to keep around from my perspective as the original author.

[mapsam]

Sounds good @springmeyer - it'll be good to have some demo/example code around, but right now the vector-tile decoder feels almost more complex than decoding with vtzero, since we've removed the helpful methods from the 1.x branch. Not sure what the long-term plan is here, but I'm sure @flippmoke has an idea! I'm going to remove the demo code from this branch now, so we can start fresh with 2.x