Make some updates to the README

[rmglennon]

We need to make a few updates to the README, such as:

• links to formatted version on mapzen.com
• new APIs like isochrone, map-matching, and so on (or remove them here to keep it easier to update) and Mobility Explorer
• contribution instructions
• links to other repositories that no longer exist

[alexey-gl]

Dear @rmglennon
Some links to local repository are broken unfortunately.
For example:

• Optimized Route API Reference
• Why Tiles?

"Technical descriptions available in other Valhalla repositories includes" links also sounds very promising, but also broken.
Looking forward to find documents by these links.

[rmglennon]

@alexey-gl Those files are here:

https://github.com/valhalla/valhalla-docs/blob/master/optimized/api-reference.md
https://github.com/valhalla/valhalla/blob/master/docs/mjolnir/why_tiles.md

Pull requests welcomed if you'd like to make some contributions to the readme.