Add a real README.
Add a real README.

README has instructions to build and install wal2json. Also, there are
examples of use.

eulerto/wal2json,fabriziomello/wal2json,GambitResearch/replisome,dvarrazzo/wal2json,GambitResearch/replisome,dmonroy/wal2json,GambitResearch/replisome