Create ISSUE_TEMPLATE.md & PULL_REQUEST_TEMPLATE.md (dotnet#12)

* Create ISSUE_TEMPLATE.md

* Create PULL_REQUEST_TEMPLATE.md

ISSUE_TEMPLATE.md

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+### System information
+
+- **OS version/distro**:
+- **.NET Version (eg., dotnet --info)**: 
+
+### Issue
+
+- **What did you do?**
+- **What happened?**
+- **What did you expect?**
+
+### Source code / logs
+
+Please paste or attach the code or logs or traces that would be helpful to diagnose the issue you are reporting.

PULL_REQUEST_TEMPLATE.md

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+We are excited to review your PR.
+
+So we can do the best job, please check:
+
+- [ ] There's a descriptive title that will make sense to other developers some time from now. 
+- [ ] There's associated issues. All PR's should have issue(s) associated - unless a trivial self-evident change such as fixing a typo. You can use the format `Fixes #nnnn` in your description to cause GitHub to automatically close the issue(s) when your PR is merged.
+- [ ] Your change description explains what the change does, why you chose your approach, and anything else that reviewers should know.
+- [ ] You have included any necessary tests in the same PR.