Add CONTRIBUTING.md

Author: p12tic

CONTRIBUTING.md

@@ -0,0 +1,45 @@
+Contributions to the library are gladly accepted.
+
+The main development happens in https://github.com/p12tic/libsimdpp repository.
+
+If you don't have time to submit a bug fix, please open an issue report on
+GitHub so at least the problem is known. The same applies to improvements -
+don't hesitate to open feature request so that the design of new APIs could
+take your use case into account.
+
+Documentation
+-------------
+
+The documentation is written on a MediaWiki instance which is currently private.
+If you find a problem in the documentation, please open an issue with a link to
+the page in question.
+
+Code changes
+------------
+
+Please test your code changes by running the following:
+
+```
+mkdir build
+cd build
+cmake ..
+
+# on Linux / macOS
+make check -j8
+
+# on Windows
+cmake --build . --target check
+```
+
+Please use the `dev` branch as a target for pull requests. The reason for this
+is that the public continuous integration services used by the library cover
+only SSE2-AVX2 instruction sets. The rest are periodically tested on a private
+test farm. Due to the number of tested compilers and library configurations
+this testing is usually done only on each merge from the `dev` branch to
+the `master` branch.
+
+Code style
+----------
+
+C-style pointer casts are discouraged, use reinterpret_cast for that. For
+non-pointer types C-style casts are preferred over static_cast or constructor.

README.md

@@ -101,6 +101,11 @@ simdpp/detail/workarounds.h for more details.
 
  - MSA is not supported on GCC 6.x and older.
 
+Contributing
+------------
+
+Contributions are welcome. Please see CONTRIBUTING.md for more information.
+
 License
 -------