Update READMEs.

Author: trekhleb

README.md

@@ -96,8 +96,9 @@ Source: [Big O Cheat Sheet](http://bigocheatsheet.com/).
 ![Big O of sorting algoritms](https://github.com/trekhleb/javascript-algorithms/blob/master/assets/big-o-sorting-algorithms.png)
 Source: [Big O Cheat Sheet](http://bigocheatsheet.com/).
 
-| Name                  | Best    | Average   | Worst   | Memory    | Stable    | Method        | Notes           |
-| --------------------- | :-----: | :-------: | :-----: | :-------: | :-------: | :------------ | :-------------- |
-| **Bubble sort**       | n       | n^2       | n^2     | 1         | Yes       | Exchanging    | Tiny code size  |
-| **Insertion sort**    | n       | n^2       | n^2     | 1         | Yes       | Insertion     | O(n + d), in the worst case over sequences that have d inversions |
-| **Selection sort**    | n^2     | n^2       | n^2     | 1         | No        | Selection     | Stable with O(n) extra space, for example using lists |
+| Name                  | Best      | Average   | Worst     | Memory    | Stable    | Method        | Notes           |
+| --------------------- | :-------: | :-------: | :-------: | :-------: | :-------: | :------------ | :-------------- |
+| **Bubble sort**       | n         | n^2       | n^2       | 1         | Yes       | Exchanging    | Tiny code size  |
+| **Insertion sort**    | n         | n^2       | n^2       | 1         | Yes       | Insertion     | O(n + d), in the worst case over sequences that have d inversions |
+| **Selection sort**    | n^2       | n^2       | n^2       | 1         | No        | Selection     | Stable with O(n) extra space, for example using lists |
+| **Heap sort**         | n log(n)  | n log(n)  | n log(n)  | 1         | No        | Selection     |                 |

src/algorithms/sorting/bubble-sort/README.md

@@ -8,3 +8,7 @@ The pass through the list is repeated until no swaps
 are needed, which indicates that the list is sorted.
 
 ![Algorithm Visualization](https://upload.wikimedia.org/wikipedia/commons/c/c8/Bubble-sort-example-300px.gif)
+
+## References
+
+[Wikipedia](https://en.wikipedia.org/wiki/Bubble_sort)

src/algorithms/sorting/insertion-sort/README.md

@@ -10,3 +10,6 @@ sort.
 
 ![Algorithm Visualization](https://upload.wikimedia.org/wikipedia/commons/0/0f/Insertion-sort-example-300px.gif)
 
+## References
+
+[Wikipedia](https://en.wikipedia.org/wiki/Insertion_sort)

src/algorithms/sorting/selection-sort/README.md

@@ -12,3 +12,7 @@ memory is limited.
 ![Algorithm Visualization](https://upload.wikimedia.org/wikipedia/commons/b/b0/Selection_sort_animation.gif)
 
 ![Algorithm Visualization](https://upload.wikimedia.org/wikipedia/commons/9/94/Selection-Sort-Animation.gif)
+
+## References
+
+[Wikipedia](https://en.wikipedia.org/wiki/Selection_sort)