Update README.md about MVT ids

[zbycz]

Regarding resolution here:

#157 (comment)
Closed with {vector tile feature id} = {original ID} * {feature_source_id_multiplier arg value} + {1 for OSM nodes, 2 for OSM ways, 3 for OSM relations, 0 for any other source} with a default feature-source-id-multiplier=10.

I propose to update documentation accordingly.

[phanecak-maptiler]

Thank you. Merging.