all: add a doc.go file for high level intro and dive for packages

[odeke-em]

Raised offline, let's add a doc.go file to each package whenever we can, to share some high level intro into our packages. I highly believe this will improve developer productivity.

I got bit by the lack of context/familiarity with the libraries in tendermint/go-crypto#35 which supersedes tendermint/go-crypto#32 and tendermint/go-crypto#31, in which I spent a bit of time extracting the code from the libraries yet actually it only required invoking GenEd25519Key().PubKey()

My offline exhibit was:
If so, we can do such a thing as https://github.com/golang/go/blob/0202aa8ec79bf700732812624eedcec3e6e300ad/src/net/http/doc.go#L5-L107 which then gives the Overview at https://golang.org/pkg/net/http/
which gives

https://godoc.org/github.com/golang/go/src/net/http

[melekes]

I second that. Let's try to document packages better!