docs: add hotplate problem

hotplate/README.md

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+HotPlate
+===
+
+(originally from <https://gist.github.com/coolaj86/6033171>)
+
+You have a 16x16 grid of which the 4 centermost cells are always 100 degrees and the cornermost cells are always 0 degrees. All other cells start at 50 degrees.
+
+For example, a 6x6 grid would look like this:
+
+    |  0  | 50  | 50  | 50  | 50  |  0  |
+    | 50  | 50  | 50  | 50  | 50  | 50  |
+    | 50  | 50  | 100 | 100 | 50  | 50  |
+    | 50  | 50  | 100 | 100 | 50  | 50  |
+    | 50  | 50  | 50  | 50  | 50  | 50  |
+    |  0  | 50  | 50  | 50  | 50  |  0  |
+
+Each turn the temperature of each cell (aside from the 4 hot and 4 cold) changes to be the average of the 4 adjacent cells left, right, up, and down.
+
+The goal is to determine how many turns it takes until not a single cell has changes more than 0.001 degrees and then print out the grid.
+
+For example, the result of running on a 6x6 grid would be this:
+
+    Grid: 6x6
+    Turns: 57
+    Diff: 0.001
+       0.00  33.33  49.99  49.99  33.33   0.00
+      33.33  50.00  66.66  66.66  50.00  33.33
+      49.99  66.66 100.00 100.00  66.66  49.99
+      49.99  66.66 100.00 100.00  66.66  49.99
+      33.33  50.00  66.66  66.66  50.00  33.33
+       0.00  33.33  49.99  49.99  33.33   0.00
+    Actual Diff: 0.0009489096904715666
+
+Or with a splash of color:
+
+!["Fancy Grid"](http://i.imgur.com/udnNfE6.png "Fancy 6x6")
+
+**Bonus**:
+Print out a colorful web page grid using jQuery.
+You'll need
+[a function to convert from wavelength to rgb](https://github.com/scottbyrns/Wavelength-To-RGB/blob/master/wavelength.js).
+Instead of a tight loop, only do one turn per millisecond and update the grid each turn so that you can watch it update (or 10 milliseconds, whatever number seems visually appealing).
+
+Hint
+---
+
+Many find it conceptually simpler to use 2-dimensional arrays for this problem, however, it's easier to solve programmatically with a 1-dimensional array. Either approach will work. The former is easier to understand, the second is easier to code.
+
+(more hints to come)