Add a contributors file.

Author: andrewdavidmackenzie

CONTRIBUTORS.md

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+Contributors to libproc-rs 
+=
+
+[<img alt="andrewdavidmackenzie" src="https://avatars3.githubusercontent.com/u/5529132?s=460&v=4" width="117">]()
+
+[andrewdavidmackenzie](https://github.com/andrewdavidmackenzie)
+
+[<img alt="lambda-fairy" src="https://avatars3.githubusercontent.com/u/1572323?s=400&v=4" width="117">]()
+
+[lambda-fairy](https://github.com/lambda-fairy)
+
+[<img alt="LuoZijun" src="https://avatars1.githubusercontent.com/u/717717?s=400&v=4" width="117">]()
+
+[LuoZijun](https://github.com/LuoZijun)