Update our language on our policy applying to security issues (PowerShell#10304)

Author: SteveL-MSFT

.github/SECURITY.md

@@ -1,5 +1,12 @@
 # Security Vulnerabilities
 
+Security issues are treated very seriously and will, by default,
+takes precedence over other considerations including usability, performance,
+etc...  Best effort will be used to mitigate side effects of a security
+change, but PowerShell must be secure by default.
+
+## Reporting a security vulnerability
+
 If you believe that there is a security vulnerability in PowerShell,
 it **must** be reported to [[email protected]](https://technet.microsoft.com/security/ff852094.aspx) to allow for [Coordinated Vulnerability Disclosure](https://technet.microsoft.com/security/dn467923).
 **Only** file an issue, if [[email protected]](https://www.microsoft.com/en-us/msrc/faqs-report-an-issue?rtc=1) has confirmed filing an issue is appropriate.