Add in introduction a brief terminology paragraph

[herrBez]

Add in the introduction a brief terminology paragraph, in which we say that for geth we intend go-ethereum implemnetation\footnote{url to go ethereum} and for hash we intend always the keccak256...

[gfornari]

For the hash, we could add a footnote at the first occurrence of it (in Kademlia protocol), like:

hash\footnote{With "hash" we always intend the Keccak-256 hash.}

[herrBez]

We can do the same for geth in my opinion, so we could also close #42

[gfornari]

Good

[herrBez]

It's already so :sunglasses: