added contributing policy

CONTRIBUTING.md

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+# Contributing
+
+Feel free to submit an issue or pull request. Each change will be considered based on the style and technical merits of the submitted solution or suggestion.
+
+## Issues
+
+When creating an issue, please be sure to include the following to ensure that your question can be answered:
+
+* A detailed description of the issue or question
+* Steps taken to try and solve the issue or answer the question independently. If you are unsure of any steps to attempt, please say so.
+* The version of Go being used
+* The ```go-sortedmap``` commit ID of the code being used
+* A complete, working, snippet of code that demonstrates the issue being brought up
+
+## Pull Requests
+
+* Ensure that your changes will benefit the project's userbase and will fit its purpose, without venturing out of scope.
+* Make sure that the coding style of submitted pull requests matches the established coding style.
+* Please do not submit pull requests that fail existing tests.
+* Please create new tests for submitted work in order to keep code coverage at 100%.
+
+Thank you for reading and considering this policy.