Added breaking change template (#383)

.github/ISSUE_TEMPLATE/breaking-change.md

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+---
+name: ASP.NET Core breaking change
+about: Report a breaking change in ASP.NET Core
+
+---
+<!--
+This issue template is for use in opening issues that document breaking changes. This template should be used to create an issue by Microsoft product team members who are documenting a breaking change.
+
+Bracketed text is a placeholder; replace the text with the requested information and remove the brackets before submitting the issue. Also, remove this comment before submitting the issue.
+
+-->
+## [Change Title]
+
+[Brief description of the change]
+
+### Version introduced
+
+[Version in which the breaking change first occurred (for example, 3.0 for ASP.NET Core 3.0)]
+
+### Old behavior
+
+### New behavior
+
+### Reason for change
+
+### Recommended action
+
+[ Suggested steps if user is affected go here:
+- Possible workarounds
+- Example of code changes to handle change
+]
+
+### Category
+
+ASP.NET
+
+### Affected APIs
+
+[ If no APIs are affected, this should read:
+     "Not detectable via API analysis"
+
+  If affected APIs are identifiable, include a link for each. The link takes the form:
+
+  `[friendly description of API](link to API on docs.microsoft.com)`
+
+  For example, `[String.IndexOf(String)](https://docs.microsoft.com/dotnet/api/system.string.indexof#System_String_IndexOf_System_String_)
+
+  For methods, if all overloads are affected, link to the general overloaded method page. For example:
+
+  <https://docs.microsoft.com/dotnet/api/system.string.indexof>
+
+  Otherwise, link to the individual method overload. For example:
+
+  <https://docs.microsoft.com/dotnet/api/system.string.indexof#System_String_IndexOf_System_String_>
+
+]
+
+<!-- Do not modify anything below this line -->
+
+---
+#### Issue metadata
+
+* Issue type: breaking-change