Explain trading in loading screen

[elexis]

Even experienced players don't know how to trade efficiently. We should add some info to the tips that are shown in the loading screen (can be found in binaries/data/mods/public/gui/text/tips):

Usability:

• Killing trade carts loots the resources they carry.
• The routes trade carts use can be reassigned by selecting trade carts and then rightclicking two markets.
• The ratio of gathered resources can be set in the trading dialog at the top left.
• The ally will get 25% of the resources when trading with his market.

Efficiency:

• The profit increases with the distance, thus markets should be built as far apart as possible.
• At some distance trade carts generate income faster than the same amount of men gathering resources.
• Trading with carts from a market to a dock helps with increasing the distance.
• Since producing a trade cart costs 80 metal, killing some of them can be more damaging to the enemy than destroying a civic centre!

This is more than fits into a single of these txt-files, so splitting it in two files might be advisable.

Notice the file also contains a name of an image that will be displayed.