Added CONTRIBUTING.md document

Author: coot

CONTRIBUTING.md

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+# Contributing to io-sim
+
+We designed `io-classes` to be as close as possible to what `base` package
+provides.  Almost all `IO` instances instantiate with api provided by one of
+the core packages, see
+[example](https://github.com/input-output-hk/io-sim/blob/main/io-classes/src/Control/Monad/Class/MonadSTM.hs?plain=1#L410-L446).
+Please keep this in mind when adding new functionality.
+
+## Style Guide
+
+Please follow the [style guide][style-guide].
+
+## Pull Request
+
+Ideally the series of commits you want to include should be written with the
+reviewer in mind.  This will make it easier for the maintainers to review your
+changes.  Each commit should heave a clear explanation what it changes and most
+importantly why the changes are necessary.   We prefer if commits are small and
+just introduce a single change.  For example if you are adding a new typeclass
+in `io-classes` and you want `io-sim` and `io-sim-por` to support it, we'd
+expect at least three commits: first which introduces the new type class, and
+separate commits which add implement it for `IOSim` and `IOSimPOR`, since both
+are using the same free monad, just interpret it differently, it's ok to first
+implement it for `IOSim` and leave placeholders in the `IOSimPOR` interpreter.
+
+If you are adding a new functionality to `MonadSTM`, don't forget to support it
+in `strict-stm` package.
+
+Please also include your changes in `CHANGELOG.md` file.
+
+
+[style-guide]: https://github.com/input-output-hk/ouroboros-network/blob/master/docs/StyleGuide.md
+