Add repository for type definitions

[stereobooster]

How about to have repository for type definitions like https://github.com/flowtype/flow-typed

Real world usage: as soon as we get signatures for std lib and top gems it can be used for real world projects.

Also: plum-umd/rtc#18

UPD: found that there are definitions directly in the project

[jeffrey-s-foster]

Sorry for the radio silence on this.

Right, for now the type definitions are just stored in the lib/types directory. If the set of types grows a lot it may be good to factor them out to a different repo in the future. But for now I'm planning to just leave them with the rdl code for simplicity.