Create CONTRIBUTING.md

Adding instructions for external contribution.

CONTRIBUTING.md

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+# Instructions for Contributing Code
+
+## Contributing bug fixes and features
+SIDH is currently accepting contributions in the form of bug fixes and new features. Any submission must have an issue tracking it in the issue tracker that has been approved by the SIDH team. Your pull request should include a link to the bug that you are fixing. If you've submitted a PR for a bug, please post a comment in the bug to avoid duplication of effort.
+
+## Legal
+If your contribution is more than 15 lines of code, you will need to complete a Contributor License Agreement (CLA). Briefly, this agreement testifies that you are granting us permission to use the submitted change according to the terms of the project's license, and that the work being submitted is under appropriate copyright.
+
+Please submit a Contributor License Agreement (CLA) before submitting a pull request. You may visit https://cla.microsoft.com to sign digitally. Alternatively, download the agreement ([Microsoft Contribution License Agreement.docx](https://www.codeplex.com/Download?ProjectName=typescript&DownloadId=822190) or [Microsoft Contribution License Agreement.pdf](https://www.codeplex.com/Download?ProjectName=typescript&DownloadId=921298)), sign, scan, and email it back to cla@microsoft.com. Be sure to include your github user name along with the agreement. Once we have received the signed CLA, we'll review the request.