Add README.

Author: ChrisHines

README.md

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+[![GoDoc](https://godoc.org/gopkg.in/stack.v0?status.svg)](https://godoc.org/gopkg.in/stack.v0)
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+Package stack implements utilities to capture, manipulate, and format call stacks. It provides a simpler API than package runtime.
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+The implementation takes care of the minutia and special cases of interpreting the program counter (pc) values returned by runtime.Callers.
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+Package stack's types implement fmt.Formatter, which provides a simple and flexible way to declaratively configure formatting when used with logging or error tracking packages.