Create SECURITY.md and issue template for vulnerabilities

Author: dnperfors

.github/ISSUE_TEMPLATE/vulnerability.md

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+---
+name: Security vulnerability
+about: Create a report about a vulnerability
+title: ''
+labels: 'security vulnerability'
+assignees: ''
+
+---
+
+**Describe the vulnerability**
+A clear and concise description of what the vulnerability is.
+
+**To Reproduce**
+Code example that reproduces the behavior:
+```
+[Fact]
+public void Issue()
+{
+...
+}
+```
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Background (please complete the following information):**
+ - OS: [e.g. iOS]
+ - Version of library: [e.g. 22]
+ - Version of dotnet core: [e.g. 3.1]
+
+**Additional context**
+Add any other context about the vulnerability here.

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Currently only the latest version of NuGet packages will receive vulnerability updates.
+
+## Reporting a Vulnerability
+
+Vulnerabilities in the libraries can be reported via an issue on GitHub. A PR to fix the issue is always welcome.