Updated README.

README.md

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+Coding Challenge: The Luhny Bin
+===============================
+
+> "To err is human; to forgive, divine." -Alexander Pope
+
+Computers break. Mistakes happen. At Square, we treat human error as a given. We anticipate potential slip-ups and implement safety measures to mitigate—and oftentimes completely eliminate—any repercussions.
+
+For example, Square's Luhn filter monitors logs and masks anything that looks like a credit card number. If a number like "4111 1111 1111 1111" were accidentally logged as part of an error message, our filter would replace it with "XXXX XXXX XXXX XXXX."
+
+The Luhn filter looks for sequences of digits that pass <a href="http://en.wikipedia.org/wiki/Luhn_algorithm">the Luhn check</a>, a simple checksum algorithm invented by Hans Peter Luhn in 1954. All valid credit card numbers pass the Luhn check, thereby enabling computer programs, like our log filter, to distinguish credit card numbers from random digit sequences.
+
+The Luhn check works like this:
+
+1. Start from the *rightmost* digit and work left.
+2. Double every second digit.
+3. If a product has two digits, treat the digits independently.
+4. Sum each individual digit, including the non-doubled digits.
+5. Divide the result by 10.
+6. If the remainder is 0, the number passed the Luhn check.
+
+For example, "5678" passes the Luhn check:
+
+1. Double every other digit: 10, 6, 14, 8
+2. Sum the individual digits: (1 + 0) + 6 + (1 + 4) + 8 = 20
+3. Divide the result by 10: 20 mod 10 = 0 **(Pass)**
+
+"6789" does not:
+
+1. Double every other digit: 12, 7, 16, 9
+2. Sum the individual digits: (1 + 2) + 7 + (1 + 6) + 9 = 26
+3. Divide the result by 10: 26 mod 10 != 0 **(Fail)**
+
+**Now for the challenge.**
+
+Write a program that reads data from standard input, masks sequences of digits that look like credit card numbers, and writes the filtered data to standard output. For the purposes of this challenge, a credit card number:
+
+- Is between 14 and 16 characters long, inclusive.
+- Passes the Luhn check.
+- Can contain any combination of `' '` or `'-'` characters.
+
+If a sequence of digits looks like a credit card number, replace each digit with an `'X'`. Any characters, including digits, may flank a credit card number. *Beware:* A valid 16-digit number can contain a valid 14 or 15-digit number. Your program must mask every digit.
+
+This isn't a contest, but an innovative solution could score you interviews at Square. I'm primarily interested to see how different programming languages stack up with regard to readability and performance.
+
+I already wrote a test suite, so you can jump straight to the fun part: writing the algorithm. To participate:
+
+1. Fork the [Luhny Bin GitHub repo](https://github.com/square/luhnybin).
+2. Modify `mask.sh` to call your program.
+3. Test your program by executing `run.sh`.
+4. Once `run.sh` passes, post a link to your solution in the comments on [our blog](http://corner.squareup.com/).
+
+Please make it easy for someone else to check out and run your solution.
+
+The test suite outputs ASCII-encoded data. It expects the same for input. If you pass a number on the command line, `run.sh` will repeat the test suite the specified number of times; this is useful for performance comparisons. The tests aren't set in stone&mdash;if you have an idea for improving the test suite, please submit a pull request. Windows users should use [Cygwin](http://www.cygwin.com/) to run the tests.
+
+Once we have enough interesting submissions, I'll post a followup to [our blog](http://corner.squareup.com/) summarizing the results. Good luck!