Signatures should accompany transaction examples for the asset tutorial

[ozgunozerk]

Issue an asset tutorial: transactions are shown, but signature part is not shown. Signing part would be helpful due to:

• at first, I couldn’t understand why the operations were secure (for example trustline creation), because only public keys were used. I thought “anybody can create a trustline for any account then?”.
• Signing part would also convey the message who is initiating and responsible from the action (transaction).