Added basic description of semigroup. Fixed comonad

readme.md

@@ -321,9 +321,9 @@ Extract takes a value out of a container. Essentially it's the opposite of `of`.
 CoIdentity(1).extract() // 1
 ```
 
-Extend runs a function on the comonad. The function should return the same type as the value in the Comonad. It's the opposite of `chain`.
+Extend runs a function on the comonad. The function should return the same type as the Comonad.
 ```js
-CoIdentity(1).extend(co => co.extract() + 1) // 2
+CoIdentity(1).extend(co => co.extract() + 1) // CoIdentity(2)
 ```
 ---
 
@@ -384,6 +384,12 @@ Array.prototype.equals = arr => {
 
 ## Semigroup
 
+An object that has a `concat` function that combines it with another object of the same type.
+
+```js
+[1].concat([2]) // [1, 2]
+```
+
 ---
 
 ## Foldable