README is outdated

[honzajavorek]

It contains links to non-existing docs etc. It should undergo some facelift. Issues reported by @XVincentX.

[XVincentX]

A good starting point would be to move relishapp content into the readme.

[honzajavorek]

@XVincentX I made some changes in how the docs are handled, but due to the nature of its contents, most of the valuable information will be on Relish now.

At the same time, I'd like to replace Relish with something else in the future - apiaryio/gavel-spec#21. Once #74 gets merged, I'd like to hear your feedback on whether the docs improved at least a bit and are more understandable.