updated readme

README.md

@@ -17,8 +17,30 @@ Or install it yourself as:
     $ gem install financial_maths
 
 ## Usage
+ ```ruby
+	require "financial_maths"
+
+	include FinancialMaths
+
+	calculate = Credit.new
+	```
+
+### Calculate Fixed payment equity
+	A credit in a period of 15 years, the amount is $100.000.000 and the interest is 1.44594763%, 
+	call the method following the next instruction.
+	
+	calculate.fixed_payment_equity(15,100000000,0.0144594763)
+
+	The result is a hash with the plan of payments, it looks like that
+
+	[{:period=>0, :monthly_payment=>nil, :interest=>nil, :payment=>nil, :balance=>100000000}, 
+	{:period=>1, :payment=>2001502.63, :interest=>1445947.63, :monthly_payment=>555555, :balance=>99444445},
+	{:period=>2, :payment=>1993469.5956441534, :interest=>1437914.5956441534, :monthly_payment=>555555, :balance=>98888890},
+	...
+	..
+	.
+	]
 
-TODO: Write usage instructions here
 
 ## Contributing