lnd: add SECURITY.MD to bolster security entry in README

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+The last major lnd 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 security@lightning.engineering (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`.