Add a note regarding transactions with max_fee=0 in fees.rst

[segfaultxavi]

In my initial experiments I tried to return the mosaics that the faucet gave me back to it without paying any fee. I set max_fee to 0. It has been a while (about 30 minutes) and the transaction still has not been approved:

$ symbol-cli transaction info --hash 1F8FA316A22187A36BABFFCF9DEE6F2EEBE350FD6AB4C4E32A5FD906FA472148
ERR {"statusCode":404,"statusMessage":"Not Found","body":"{\"code\":\"ResourceNotFound\",\"message\":\"no resource exists with id '1F8FA316A22187A36BABFFCF9DEE6F2EEBE350FD6AB4C4E32A5FD906FA472148'\"}"}

And I still have all my mosaics.

This is obviously my mistake, but I think this is a common enough case to warrant a note on the Fees page saying:

• What will happen if you do this?
• Will the transaction ever be approved or will it be eventually forgotten?
• Why are 0-fee transactions allowed then?
• Reminder of the philosophy behind blockchain: you have to pay for it to work (is this explained someplace else?)

[segfaultxavi]

Apparently there's a minimum fee below which nodes will reject transactions. They do not seem to report back this rejection, though.