Add issue templates (#154)

* Add issue templates

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Report a bug in this library
+title: ''
+labels: bug
+assignees: ''
+
+---
+
+<!--
+This form is for bug reports for this library only.
+For general, language-agnostic questions and troubleshooting, please ask/look for answers [here](https://groups.google.com/g/adwords-api).
+
+Please check existing issues to see if your bug has already been reported before creating a new issue. We encourage you to add any relevant information to the existing issue if one already exists.
+-->
+
+**Which version of the client library are you using?**
+
+**Which version of .NET are you using?**
+
+**Which operating system (Linux, Windows, ...) and version?**
+
+**Actions taken**
+Please provide a code snippet or steps to consistently reproduce the experienced behavior.
+[DO NOT INCLUDE ANY PERSONAL OR SENSITIVE DATA IN YOUR SUBMITTED CODE]
+
+**Expected result**
+
+**Actual result**
+
+Make sure you include information that can help us debug (full error message, exception listing, stack trace, logs).
+See [troubleshooting](https://developers.google.com/google-ads/api/docs/best-practices/troubleshooting) for how to diagnose problems better.
+
+**Anything else we should know about your project / environment**

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Request a new feature or improvement
+title: ''
+labels: enhancement
+assignees: ''
+
+---
+
+<!--
+This form is for feature requests for this library only.
+For general, language-agnostic questions and troubleshooting, please ask/look for answers [here](https://groups.google.com/g/adwords-api).
+
+Please check existing issues to see if your feature request has already been identified before creating a new issue. We encourage you to add any relevant information to the existing issue if one already exists.
+-->
+
+**Describe the problem you are trying to solve**
+
+**Describe the solution you would like**
+
+**Describe alternatives you've considered**
+
+**Additional context**
+Add any other context or screenshots about the feature request here.