Clarify package purpose and motivation

[pz-max]

The purpose and motivation of the package should be described at the beginning of the readme.

• I think the package purpose is to get raw osm data (almost 100% raw, just light cleaning -- as mentioned in another issue we need to show where the light cleaning is happening)
• Motivation was that pyrosm did not work for extracting data for whole continents/ or large countries (USA, CHINA, NIGERIA etc.)

[mnm-matin]

Great question!
I would add to your statements that:

• The purpose is to get raw osm data without external C dependancies + standardize cleaning (i.e common functions to join nodes and ways, cleanup names, handcrafted rules for data validation)
• the motivation is as you have stated that extracting data via apis is not reliable and prone to rate limits. We use reliable geofabrik servers....