Add issue template

.github/ISSUE_TEMPLATE.md

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+Thank you for making this library better by submitting an issue. To ensure we can troubleshoot effectively,
+please fill out this form in its entirety.
+
+If you are submitting a feature request you may ignore this template and enter a free-form description of your request.
+You may also contact the maintainers directly via the Nexmo Slack to discuss the feature.
+
+### General description of the issue or question
+(Please be as descriptive as possible. If this is a general Nexmo question (billing, usage, etc.) you
+will receive a better experience by either [filing a support request](https://help.nexmo.com/hc/en-us/requests/new)
+directly to Nexmo or asking in the [Nexmo Slack](https://developer.nexmo.com/community/slack).)
+
+### Which target framework(s) of .NET can you reproduce this issue against?
+(Examples: .NET Core 2.0, netcoreapp1.0, .NET Framework 4.6.2, etc.)
+
+### What operating system(s) is exhibiting this issue?
+
+### Version or commit hash
+Which version(s) of `CSharp.Nexmo.Client` did you encounter this issue?
+
+### Expected Behavior
+What should have happened?
+
+### Actual Behavior
+What actually happened?
+
+### Steps to Reproduce
+Please provide a code snippet or steps to reproduce the issue.