Adding README to individual project.

P007-Reverse-Integer/README.md

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+# Reverse Integer
+
+## Instructions
+
+Reverse digits of an integer.
+
+**Example1**: x = 123, return 321
+
+**Example2**: x = -123, return -321
+
+**Note**:
+The input is assumed to be a 32-bit signed integer. Your function should return 0 when the reversed integer overflows.