Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to `mp-units`
+
+👍🎉 First off, thanks for taking the time to contribute! 🎉👍
+
+## Download, Build, Install
+
+Please refer to our official docs for [download, build, and install instructions](https://mpusz.github.io/units/usage.html).
+
+## Where to start?
+
+If you are looking for a good issue to start with, please check the following:
+- [good first issue](https://github.com/mpusz/units/labels/good%20first%20issue) - issues that should be pretty simple to implement.
+- [help wanted](https://github.com/mpusz/units/labels/help%20wanted) - issues that typically are a bit more involved than beginner issues.
+- [high priority](https://github.com/mpusz/units/labels/high%20priority) - things to fix ASAP but often of higher complexity.
+
+## Code Formatting
+
+We intend to use `clang-format` when it finally learns what a C++ concept is :wink:.
+
+Until then, please code alike what is there already:
+- 2 spaces for indentation
+- no indentation for namespaces
+- types, functions, variables naming in a `standard_case`
+- template parameters in a `PascalCase`
+
+## Backward Compatibility
+
+Before submission, please remember to check if the code compiles fine on all the supported compilers (especially gcc-9.3 and MSVC are tricky).
+Unfortunately, we cannot add gcc-9.3 to the CI process as it is [not supported by Conan Docker images](https://github.com/conan-io/conan-docker-tools/issues/200).