Added CONTRIBUTING.md

CONTRIBUTING.md

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+Thank you for your contribution!
+
+Please be aware that RaptorJIT is [Apache 2.0 licensed](https://www.apache.org/licenses/LICENSE-2.0)
+and when you contribute code you are also releasing it under the same
+license. If this is a problem, for example because you are unwilling
+or unable to release your code under this license, then please explain
+the situation in your pull request so that we can find an appropriate
+solution.
+
+Please do not put individual copyright notices into source files. This
+leads to clutter over time and complicates refactoring. If you own the
+copyright of your contribution then this will be implicit in the Git
+history (as clarified in the COPYING/COPYRIGHT files) and no notice is
+needed. If you are not the copyright owner, perhaps due to a copyright
+assignment agreement with your employer, then please mention this in
+your Pull Request and we will add an appropriate clarification to the
+COPYRIGHT file.
+
+Please also respect the
+[Contributor Covenent Code of Conduct](http://contributor-covenant.org/version/1/4/) 
+in your interactions with the RaptorJIT community.