btcd: add SECURITY.md

Author: Roasbeef

SECURITY.md

@@ -0,0 +1,15 @@
+# Security Policy
+
+## Supported Versions
+
+The last major `btcd` release is to be considered the current support version.
+Given an issue severe enough, a backport will be issued either to the prior
+major release or the set of releases considered utilized enough. 
+
+## Reporting a Vulnerability
+
+To report security issues, send an email to [email protected]
+(this list isn't to be used for support). 
+
+The following key can be used to communicate sensitive information: `91FE 464C
+D751 01DA 6B6B  AB60 555C 6465 E5BC B3AF`.