Update README.

Author: trekhleb

README.md

@@ -69,7 +69,7 @@
   * [Dijkstra Algorithm](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/dijkstra) - finding shortest path to all graph vertices
   * [Bellman-Ford Algorithm](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/bellman-ford) - finding shortest path to all graph vertices
   * [Detect Cycle](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/detect-cycle) - for both directed and undirected graphs (DFS and Disjoint Set based versions)
-  * Prim’s Algorithm - finding Minimum Spanning Tree (MST)
+  * [Prim’s Algorithm](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/prim) - finding Minimum Spanning Tree (MST)
   * Kruskal’s Algorithm - finding Minimum Spanning Tree (MST)
   * Topological Sorting
   * Eulerian path, Eulerian circuit
@@ -85,6 +85,7 @@
 * **Greedy**
   * [Unbound Knapsack Problem](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/sets/knapsack-problem)
   * [Dijkstra Algorithm](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/dijkstra) - finding shortest path to all graph vertices
+  * [Prim’s Algorithm](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/prim) - finding Minimum Spanning Tree (MST)
 * **Divide and Conquer**
   * [Euclidean Algorithm](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/math/euclidean-algorithm) - calculate the Greatest Common Divisor (GCD)
   * [Permutations](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/sets/permutations) (with and without repetitions)

src/algorithms/graph/prim/README.md

@@ -0,0 +1,46 @@
+# Prim's Algorithm
+
+In computer science, **Prim's algorithm** is a greedy algorithm that 
+finds a minimum spanning tree for a weighted undirected graph. 
+
+The algorithm operates by building this tree one vertex at a 
+time, from an arbitrary starting vertex, at each step adding 
+the cheapest possible connection from the tree to another vertex.
+
+![Prim's Algorithm](https://upload.wikimedia.org/wikipedia/commons/f/f7/Prim%27s_algorithm.svg)
+
+Prim's algorithm starting at vertex `A`. In the third step, edges 
+`BD` and `AB` both have weight `2`, so `BD` is chosen arbitrarily. 
+After that step, `AB` is no longer a candidate for addition 
+to the tree because it links two nodes that are already 
+in the tree.
+
+## Minimum Spanning Tree
+
+A **minimum spanning tree** (MST) or minimum weight spanning tree 
+is a subset of the edges of a connected, edge-weighted 
+(un)directed graph that connects all the vertices together, 
+without any cycles and with the minimum possible total edge 
+weight. That is, it is a spanning tree whose sum of edge weights 
+is as small as possible. More generally, any edge-weighted 
+undirected graph (not necessarily connected) has a minimum 
+spanning forest, which is a union of the minimum spanning 
+trees for its connected components.
+
+![Minimum Spanning Tree](https://upload.wikimedia.org/wikipedia/commons/d/d2/Minimum_spanning_tree.svg)
+
+A planar graph and its minimum spanning tree. Each edge is 
+labeled with its weight, which here is roughly proportional 
+to its length.
+
+![Minimum Spanning Tree](https://upload.wikimedia.org/wikipedia/commons/c/c9/Multiple_minimum_spanning_trees.svg)
+
+This figure shows there may be more than one minimum spanning 
+tree in a graph. In the figure, the two trees below the graph 
+are two possibilities of minimum spanning tree of the given graph.
+
+## References
+
+- [Minimum Spanning Tree on Wikipedia](https://en.wikipedia.org/wiki/Minimum_spanning_tree)
+- [Prim's Algorithm on Wikipedia](https://en.wikipedia.org/wiki/Prim%27s_algorithm)
+- [Prim's Algorithm on YouTube by Tushar Roy](https://www.youtube.com/watch?v=oP2-8ysT3QQ)