Updated installation procedure.

[juanmed]

Hello, thanks for sharing this simulator. After some headache trying to run it, finally got it running. I suggest source installation of both dartsim and pydart2 since were the only 2 methods that worked for me. Please refer to #2 for some details.

Also added a requirements.txt for easier dependencies installation but did not included it in the readme. It is your call whether to add it or not. I can make a new commit including it if you want.

Thanks.

[juanmed]

By the way, I think any sudo apt-get upgrade will break the simulator since will also update dartsim to the newest version, which is incompatible with pydart2. Actually maybe that happened to me since some weeks ago I ran flappy without major issues. Maybe a warning on this is necessary?