docs: Add description of pingpong subject

subjects/pingpong/README.md

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+### Instructions
+
+Write a program that checks if a number up to a given number is an odd or even number.
+The number will be between `0` and `100`.
+
+- If the number is `odd` then it prints `ping` as in the example below.
+- If the number is `even` then it prints `pong`.
+- If the number of arguments is invalid then do nothing.
+
+### Usage
+
+```console
+$ go run . 6 | cat -e
+0 ping$
+1 pong$
+2 ping$
+3 pong$
+4 ping$
+5 pong$
+$ go run . | cat -e
+$
+```