readonly variables are can be initilized at compile time
but we can assign values to readonly varialble at once 
through functions or constructors

but in case of constant variables once assign a value we 
can't change value and must assign value to constant 
variables at the time of declarition

I am adding another point to the above answer.

as the constant variable is constant over the class,that 
means,all the objects created for that class have the same 
value.
but in case of readonly variables,we can assign different 
values in the different instances of the class it contains.

readonly is compile time constant.
constant is runtime constant.

constatnt field is used to initialize at the declaration of 
the field.
readonly field is used at the declaration or constructor of 
the field.