Fix the build instructions

[hexagonrecursion]

I have managed to compile the game, despite incorrect build instructions and other difficulties.

1. This three lines assume we are inside <colobot_repository>/build/ but we never changed into it:

cmake ../
make
colobot -datadir ../data

2. There was no <colobot_repository>/build/bin/ directory. The executable was in <colobot_repository>/build/

Edit: building the game and getting it to run required a lot of trial-and-error (especially the dev branch). I added more complete build instructions to reduce the trial-and-error for future contributors.

Design philosophy:

• Concrete is easier to understand than abstract
• Skipping steps you think are obvious will lead to contributors being stumped

Note: the instructions are missing a step: the installation of dependencies. I do not intend to add this step in this pull request.