Added Contracts (#127)

* Added description of contracts

* Added ES6 example

* linted and toc

readme.md

@@ -31,6 +31,8 @@ __Table of Contents__
 * [Value](#value)
 * [Constant](#constant)
 * [Functor](#functor)
+  * [Preserves identity](#preserves-identity)
+  * [Composable](#composable)
 * [Pointed Functor](#pointed-functor)
 * [Lift](#lift)
 * [Referential Transparency](#referential-transparency)
@@ -270,7 +272,20 @@ const predicate = (a) => a > 2
 
 ## Contracts
 
-TODO
+A contract specifies the obligations and guarentees of the behavior from a function or expression at runtime. This acts as a set of rules that are expected from the input and output of a function or expression, and errors are generally reported whenever a contract is violated.
+
+```js
+// Define our contract : int -> int
+const contract = (input) => {
+  if (typeof input === 'number') return true
+  throw new Error('Contract violated: expected int -> int')
+}
+
+const addOne = (num) => contract(num) && num + 1
+
+addOne(2) // 3
+addOne('some string') // Contract violated: expected int -> int
+```
 
 ## Guarded Functions