feat(usable-tokens): add details to subject

subjects/blockchain/usable-token/README.md

@@ -2,10 +2,37 @@
 
 ### Instructions
 
-- Create a Smart Contract named `UsableToken`
-- Like MinimalToken, its constructor takes as parameter an amount that is given initially to the deployer.
-- Create a function `approve(address,uint)` that allows the owner of the token to approve a spender to spend a certain amount of tokens.
-- Create a function `allowance(address,uint)` that returns the amount of tokens that a spender can spend on behalf of the owner.
+- Complete the following Smart Contract named `UsableToken`
+- Like `MinimalToken`, its constructor takes as parameter an amount that is
+  given initially to the deployer account.
+- Create a function `transfer(address, uint)` that allows the owner to transfer
+  a certain amount of tokens to the specified address.
+- Create a function `approve(address, uint)` that allows the owner of the token
+  to approve a spender to spend a certain amount of tokens.
+- The `allowance` states should keep track of the amount of tokens that a
+  spender can spend on behalf of the owner.
+
+```js
+contract UsableToken {
+    ... public accounts;
+    ... public allowance;
+
+    constructor(uint256 initialNumber) {
+        ...
+    }
+
+    function transfer(address to, uint256 amount) public {
+        ...
+    }
+
+    function approve(address spender, uint256 amount) public {
+        ...
+    }
+
+    function transferFrom(address from, address to, uint256 amount) public {
+    }
+}
+```
 
 ### Notions