Explain epoch link

[Zosoled]

Updated integration guide "The Basics" to add brief meaning of link in epoch blocks. Text summarized from comment in header file: https://github.com/nanocurrency/nano-node/blob/develop/nano/lib/epochs.hpp

/** Returns true if link matches one of the released epoch links.
 *  WARNING: just because a legal block contains an epoch link, it does not mean it is an epoch block.
 *  A legal block containing an epoch link can easily be constructed by sending to an address identical
 *  to one of the epoch links.
 *  Epoch blocks follow the following rules and a block must satisfy them all to be a true epoch block:
 *    epoch blocks are always state blocks
 *    epoch blocks never change the balance of an account
 *    epoch blocks always have a link field that starts with the ascii bytes "epoch v1 block" or "epoch v2 block" (and possibly others in the future)
 *    epoch blocks never change the representative
 *    epoch blocks are not signed by the account key, they are signed either by genesis or by special epoch keys
 */