updated generic linux build instructions

[Former user]

cd /where/i/keep/my/games/

legally obtain a gog installer and extract it to caesaria-gog-data

innoextract setup_caesar3_2.0.0.9.exe -d caesaria-gog-data

clone and build caesaria

git clone https://bitbucket.org/dalerank/caesaria.git cd caesaria cmake . make -j4

start the game you just built, using the gog c3 game data

./bin/caesaria.linux -c3gfx ../caesaria-gog-data/app -c3music ../caesaria-gog-data/wavs -c3video../caesaria-gog-data/smk