Minimal contribution guideline

Author: serge-sans-paille

CONTRIBUTING.md

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+# Contributing to xsimd
+
+First, thanks for being there! Welcome on board, We'll try to make your
+contributing journey as good an experience as it can be.
+
+# Submitting patches
+
+Patches should be submitted through Github PR. We put some effort to setup a
+decent Contibuous Integration coverage, please try to make it green ;-)
+
+We're using [clang-format](https://clang.llvm.org/docs/ClangFormat.html) to keep
+the coding style consistent, a ``.clang-format`` file is shipped within the
+source, please use it!
+
+# Extending the API
+
+We're open to extending the API, as long as it has been discussed either in an
+Issue or a PR. The only constraint is to add testing for new functions, and make
+sure they work on all supported architectures, not only your favorite one!
+
+# Licensing
+
+We use a shared copyright model that enables all contributors to maintain the
+copyright on their contributions. Stated otherwise, there's no copyright
+assignment.