Update Cash account documentation

A user recently had to deal with a situation where a large amount of cash was kept at home. His solution is worth adding to the documentation:

1. Created a "Cash Reserve" asset.
   
2. Renamed "Cash" expense to "Cash Wallet"
   
3. When I withdraw money via ATM I make a transaction from "Checking account" to "Cash Wallet"
   
4. When I pay for something with cash I make a transaction from the "Cash Wallet" expense to the appropriate expense.
   
5. When I receive an ammount of money in cash, I put it in the "Cash Reserve" asset (let's say it's a safe at my house, or a drawer, but not my wallet).
   
6. Because I have cash at home, I don't need to withdraw from my bank account, that's why when I want to spend it I make a transaction from "Cash Reserve" to "Cash Wallet", then I continue with step 4 above.