docs: add SECURITY.md (#1278)

Author: kwsantiago

SECURITY.md

@@ -0,0 +1,9 @@
+# Security
+
+Report security issues to the relevant Lightning implementation teams:
+
+- [Core Lightning](https://github.com/ElementsProject/lightning/blob/master/SECURITY.md)
+- [LND](https://github.com/lightningnetwork/lnd/blob/master/SECURITY.md)
+- [Eclair](https://github.com/ACINQ/eclair/blob/master/SECURITY.md)
+- [LDK](https://github.com/lightningdevkit/rust-lightning/blob/main/SECURITY.md)
+