Add issue template for new providers

Add a GitHub issue template for new providers.

Author: martincostello

.github/ISSUE_TEMPLATE/new_provider.md

@@ -0,0 +1,45 @@
+---
+name: New provider
+about: Suggest a new OAuth provider for AspNet.Security.OAuth.Providers
+labels: new provider
+
+---
+
+### Which provider would you like support added for?
+
+<!--
+The name of the OAuth 2.0 provider you'd like to support added for.
+Providers that deviate significantly from the OAuth 2.0 specification
+with non-standard extensions and/or behavior are unlikely to be accepted.
+-->
+
+### Provider Documentation
+
+<!--
+Any relevant links to the reference documentation for the OAuth 2.0 provider.
+These links are preferably to the technical documentation that can be worked
+from to implement the provider code with.
+-->
+
+### Are you willing to implement this provider yourself?
+
+<!--
+Are you willing to implement this provider yourself and contribute it to the
+repository as a pull request?
+
+If you are not able or willing to implement this provider yourself, the repository
+maintainers are unlikely to be able to commit to implement it for you, especially
+if the provider is not a product or service that the maintainers actively use
+and/or have access to (for example, a paid-for enterprise service).
+
+Providers that you are not able to implement yourself but would be valuable
+to the open source community if they existed will be labelled with "help wanted".
+Such issues will then be dependent on the community to implement at some point,
+but do not represent any commitment from the maintainers to do so.
+-->
+
+### Additional context
+
+<!--
+Add any other context about the new provider that may be relevant.
+-->